diff --git a/core/block_render_svg_horizontal.js b/core/block_render_svg_horizontal.js
index c1c4ecce3f..8ccc6119b0 100644
--- a/core/block_render_svg_horizontal.js
+++ b/core/block_render_svg_horizontal.js
@@ -27,6 +27,7 @@
 goog.provide('Blockly.BlockSvg.render');
 
 goog.require('Blockly.BlockSvg');
+goog.require('Blockly.constants');
 
 
 // UI constants for rendering blocks.
@@ -311,14 +312,14 @@ Blockly.BlockSvg.prototype.updateColour = function() {
   var strokeColour = this.getColourTertiary();
 
   // Render block stroke
-  this.svgPath_.setAttribute('stroke', strokeColour);
+  this.blockFrameElement_.setAttribute('stroke', strokeColour);
 
   // Render block fill
   var fillColour = (this.isGlowingBlock_) ? this.getColourSecondary() : this.getColour();
-  this.svgPath_.setAttribute('fill', fillColour);
+  this.blockFrameElement_.setAttribute('fill', fillColour);
 
   // Render opacity
-  this.svgPath_.setAttribute('fill-opacity', this.getOpacity());
+  this.blockFrameElement_.setAttribute('fill-opacity', this.getOpacity());
 
   // Bump every dropdown to change its colour.
   for (var x = 0, input; input = this.inputList[x]; x++) {
@@ -337,11 +338,11 @@ Blockly.BlockSvg.prototype.highlightForReplacement = function(add) {
   if (add) {
     var replacementGlowFilterId = this.workspace.options.replacementGlowFilterId
       || 'blocklyReplacementGlowFilter';
-    this.svgPath_.setAttribute('filter', 'url(#' + replacementGlowFilterId + ')');
+    this.blockFrameElement_.setAttribute('filter', 'url(#' + replacementGlowFilterId + ')');
     Blockly.utils.addClass(/** @type {!Element} */ (this.svgGroup_),
         'blocklyReplaceable');
   } else {
-    this.svgPath_.removeAttribute('filter');
+    this.blockFrameElement_.removeAttribute('filter');
     Blockly.utils.removeClass(/** @type {!Element} */ (this.svgGroup_),
         'blocklyReplaceable');
   }
@@ -545,12 +546,12 @@ Blockly.BlockSvg.prototype.renderDraw_ = function(metrics) {
   this.renderDrawTop_(steps, connectionsXY, metrics);
 
   var pathString = steps.join(' ');
-  this.svgPath_.setAttribute('d', pathString);
+  this.blockFrameElement_.setAttribute('d', pathString);
 
   if (this.RTL) {
     // Mirror the block's path.
     // This is awesome.
-    this.svgPath_.setAttribute('transform', 'scale(-1 1)');
+    this.blockFrameElement_.setAttribute('transform', 'scale(-1 1)');
   }
 
   // Horizontal blocks have a single Image Field that is specially positioned
diff --git a/core/block_render_svg_vertical.js b/core/block_render_svg_vertical.js
index ca5ea470f3..6a6e9b6541 100644
--- a/core/block_render_svg_vertical.js
+++ b/core/block_render_svg_vertical.js
@@ -29,6 +29,7 @@ goog.provide('Blockly.BlockSvg.render');
 goog.require('Blockly.BlockSvg');
 goog.require('Blockly.scratchBlocksUtils');
 goog.require('Blockly.utils');
+goog.require('Blockly.constants');
 
 
 // UI constants for rendering blocks.
@@ -139,7 +140,17 @@ Blockly.BlockSvg.STATEMENT_INPUT_INNER_SPACE = 2 * Blockly.BlockSvg.GRID_UNIT;
  * Height of the top hat.
  * @const
  */
-Blockly.BlockSvg.START_HAT_HEIGHT = 16;
+Object.defineProperty(Blockly.BlockSvg, 'START_HAT_HEIGHT', {
+  get: function() {
+    if (Blockly.theme === Blockly.Themes.CAT_BLOCKS) {
+      return 31;
+    }
+
+    return 16;
+  },
+  enumerable: true,
+  configurable: true
+});
 
 /**
  * Height of the vertical separator line for icons that appear at the left edge
@@ -152,7 +163,20 @@ Blockly.BlockSvg.ICON_SEPARATOR_HEIGHT = 10 * Blockly.BlockSvg.GRID_UNIT;
  * Path of the top hat's curve.
  * @const
  */
-Blockly.BlockSvg.START_HAT_PATH = 'c 25,-22 71,-22 96,0';
+Object.defineProperty(Blockly.BlockSvg, 'START_HAT_PATH', {
+  get: function() {
+    if (Blockly.theme === Blockly.Themes.CAT_BLOCKS) {
+      return 'c2.6,-2.3 5.5,-4.3 8.5,-6.2' +
+        'c-1,-12.5 5.3,-23.3 8.4,-24.8c3.7,-1.8 16.5,13.1 18.4,15.4' +
+        'c8.4,-1.3 17,-1.3 25.4,0c1.9,-2.3 14.7,-17.2 18.4,-15.4' +
+        'c3.1,1.5 9.4,12.3 8.4,24.8c3,1.8 5.9,3.9 8.5,6.1';
+    }
+
+    return 'c 25,-22 71,-22 96,0';
+  },
+  enumerable: true,
+  configurable: true
+});
 
 /**
  * SVG path for drawing next/previous notch from left to right.
@@ -476,11 +500,22 @@ Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS = 5 * Blockly.BlockSvg.GRID_UNIT;
  * SVG path for drawing the rounded top-left corner.
  * @const
  */
-Blockly.BlockSvg.TOP_LEFT_CORNER_DEFINE_HAT =
-    'a ' + Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS + ',' +
-    Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS + ' 0 0,1 ' +
-    Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS + ',-' +
-    Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS;
+Object.defineProperty(Blockly.BlockSvg, 'TOP_LEFT_CORNER_DEFINE_HAT', {
+  get: function() {
+    if (Blockly.theme === Blockly.Themes.CAT_BLOCKS) {
+      return 'c0,-7.1 3.7,-13.3 9.3,-16.9c1.7,-7.5 5.4,-13.2 7.6,-14.2' +
+        'c2.6,-1.3 10,6 14.6,11.1h33c4.6,-5.1 11.9,-12.4 14.6,-11.1' +
+        'c1.9,0.9 4.9,5.2 6.8,11.1c2.6,0,5.2,0,7.8,0';
+    }
+    
+    return 'a ' + Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS + ',' +
+        Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS + ' 0 0,1 ' +
+        Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS + ',-' +
+        Blockly.BlockSvg.DEFINE_HAT_CORNER_RADIUS;
+  },
+  enumerable: true,
+  configurable: true
+});
 
 /**
  * SVG path for drawing the rounded top-left corner.
@@ -518,8 +553,7 @@ Blockly.BlockSvg.prototype.updateColour = function() {
     }
   }
 
-  // Render block stroke
-  this.svgPath_.setAttribute('stroke', strokeColour);
+  this.blockFrameElement_.setAttribute('stroke', strokeColour);
 
   // Render block fill
   if (this.isGlowingBlock_ || renderShadowed) {
@@ -532,10 +566,10 @@ Blockly.BlockSvg.prototype.updateColour = function() {
   } else {
     var fillColour = this.getColour();
   }
-  this.svgPath_.setAttribute('fill', fillColour);
+  this.blockFrameElement_.setAttribute('fill', fillColour);
 
   // Render opacity
-  this.svgPath_.setAttribute('fill-opacity', this.getOpacity());
+  this.blockFrameElement_.setAttribute('fill-opacity', this.getOpacity());
 
   // Update colours of input shapes.
   for (var i = 0, input; input = this.inputList[i]; i++) {
@@ -567,11 +601,11 @@ Blockly.BlockSvg.prototype.highlightForReplacement = function(add) {
   if (add) {
     var replacementGlowFilterId = this.workspace.options.replacementGlowFilterId
       || 'blocklyReplacementGlowFilter';
-    this.svgPath_.setAttribute('filter', 'url(#' + replacementGlowFilterId + ')');
+    this.blockFrameElement_.setAttribute('filter', 'url(#' + replacementGlowFilterId + ')');
     Blockly.utils.addClass(/** @type {!Element} */ (this.svgGroup_),
         'blocklyReplaceable');
   } else {
-    this.svgPath_.removeAttribute('filter');
+    this.blockFrameElement_.removeAttribute('filter');
     Blockly.utils.removeClass(/** @type {!Element} */ (this.svgGroup_),
         'blocklyReplaceable');
   }
@@ -1119,6 +1153,67 @@ Blockly.BlockSvg.prototype.computeOutputPadding_ = function(inputRows) {
   row.paddingEnd += Blockly.BlockSvg.SHAPE_IN_SHAPE_PADDING[shape][otherShape];
 };
 
+// Cat face and ear animation for CatBlocks
+// TODO: Do we want to move this and `initCatFace_` to a separate file?
+// Or would just that complicate unforking?
+Blockly.BlockSvg.prototype.renderCatFace_ = function() {
+  // This only makes sense in the context of the Cat Blocks theme.
+  if (Blockly.theme !== Blockly.Themes.CAT_BLOCKS) {
+    return;
+  }
+
+  this.svgPath_.svgFace.setAttribute('fill','#000000');
+
+  var closedEye = Blockly.utils.createSvgElement('path', {}, this.svgFace_);
+  closedEye.setAttribute('d','M25.2-1.1c0.1,0,0.2,0,0.2,0l8.3-2.1l-7-4.8' +
+    'c-0.5-0.3-1.1-0.2-1.4,0.3s-0.2,1.1,0.3,1.4L29-4.1l-4,1' +
+    'c-0.5,0.1-0.9,0.7-0.7,1.2C24.3-1.4,24.7-1.1,25.2-1.1z');
+  closedEye.setAttribute('fill-opacity','0');
+  this.svgPath_.svgFace.closedEye = closedEye;
+
+  var closedEye2 = Blockly.utils.createSvgElement('path', {}, this.svgFace_);
+  closedEye2.setAttribute('d','M62.4-1.1c-0.1,0-0.2,0-0.2,0l-8.3-2.1l7-4.8' +
+    'c0.5-0.3,1.1-0.2,1.4,0.3s0.2,1.1-0.3,1.4l-3.4,2.3l4,1' +
+    'c0.5,0.1,0.9,0.7,0.7,1.2C63.2-1.4,62.8-1.1,62.4-1.1z');
+  closedEye2.setAttribute('fill-opacity','0');
+  this.svgPath_.svgFace.closedEye2 = closedEye2;
+
+  var eye = Blockly.utils.createSvgElement('circle', {}, this.svgFace_);
+  eye.setAttribute('cx','59.2');
+  eye.setAttribute('cy','-3.3');
+  eye.setAttribute('r','3.4');
+  eye.setAttribute('fill-opacity','0.6');
+  this.svgPath_.svgFace.eye = eye;
+
+  var eye2 = Blockly.utils.createSvgElement('circle', {}, this.svgFace_);
+  eye2.setAttribute('cx','29.1');
+  eye2.setAttribute('cy','-3.3');
+  eye2.setAttribute('r','3.4');
+  eye2.setAttribute('fill-opacity','0.6');
+  this.svgPath_.svgFace.eye2 = eye2;
+
+  var mouth = Blockly.utils.createSvgElement('path', {}, this.svgFace_);
+  mouth.setAttribute('d','M45.6,0.1c-0.9,0-1.7-0.3-2.3-0.9' +
+    'c-0.6,0.6-1.3,0.9-2.2,0.9c-0.9,0-1.8-0.3-2.3-0.9c-1-1.1-1.1-2.6-1.1-2.8' +
+    'c0-0.5,0.5-1,1-1l0,0c0.6,0,1,0.5,1,1c0,0.4,0.1,1.7,1.4,1.7' +
+    'c0.5,0,0.7-0.2,0.8-0.3c0.3-0.3,0.4-1,0.4-1.3c0-0.1,0-0.1,0-0.2' +
+    'c0-0.5,0.5-1,1-1l0,0c0.5,0,1,0.4,1,1c0,0,0,0.1,0,0.2' +
+    'c0,0.3,0.1,0.9,0.4,1.2C44.8-2.2,45-2,45.5-2s0.7-0.2,0.8-0.3' +
+    'c0.3-0.4,0.4-1.1,0.3-1.3c0-0.5,0.4-1,0.9-1.1c0.5,0,1,0.4,1.1,0.9' +
+    'c0,0.2,0.1,1.8-0.8,2.8C47.5-0.4,46.8,0.1,45.6,0.1z');
+  mouth.setAttribute('fill-opacity','0.6');
+
+  this.svgPath_.ear.setAttribute('d','M73.1-15.6c1.7-4.2,4.5-9.1,5.8-8.5' +
+    'c1.6,0.8,5.4,7.9,5,15.4c0,0.6-0.7,0.7-1.1,0.5c-3-1.6-6.4-2.8-8.6-3.6' +
+    'C72.8-12.3,72.4-13.7,73.1-15.6z');
+  this.svgPath_.ear.setAttribute('fill','#FFD5E6');
+
+  this.svgPath_.ear2.setAttribute('d','M22.4-15.6c-1.7-4.2-4.5-9.1-5.8-8.5' +
+    'c-1.6,0.8-5.4,7.9-5,15.4c0,0.6,0.7,0.7,1.1,0.5c3-1.6,6.4-2.8,8.6-3.6' +
+    'C22.8-12.3,23.2-13.7,22.4-15.6z');
+  this.svgPath_.ear2.setAttribute('fill','#FFD5E6');
+};
+
 /**
  * Draw the path of the block.
  * Move the fields to the correct locations.
@@ -1136,6 +1231,9 @@ Blockly.BlockSvg.prototype.renderDraw_ = function(iconWidth, inputRows) {
     // No output or previous connection.
     this.squareTopLeftCorner_ = true;
     this.startHat_ = true;
+    if (Blockly.theme === Blockly.Themes.CAT_BLOCKS) {
+      this.initCatStuff();
+    }
     inputRows.rightEdge = Math.max(inputRows.rightEdge, 100);
   }
 
@@ -1162,12 +1260,16 @@ Blockly.BlockSvg.prototype.renderDraw_ = function(iconWidth, inputRows) {
   this.renderDrawLeft_(steps);
 
   var pathString = steps.join(' ');
-  this.svgPath_.setAttribute('d', pathString);
+  this.blockFrameElement_.setAttribute('d', pathString);
+
+  if (Blockly.theme === Blockly.Themes.CAT_BLOCKS && this.startHat_ && !this.svgFace_.firstChild) {
+    this.renderCatFace_();
+  }
 
   if (this.RTL) {
     // Mirror the block's path.
     // This is awesome.
-    this.svgPath_.setAttribute('transform', 'scale(-1 1)');
+    this.blockFrameElement_.setAttribute('transform', 'scale(-1 1)');
   }
 };
 
diff --git a/core/block_svg.js b/core/block_svg.js
index 6e24dbe86f..259013de93 100644
--- a/core/block_svg.js
+++ b/core/block_svg.js
@@ -37,6 +37,7 @@ goog.require('Blockly.scratchBlocksUtils');
 goog.require('Blockly.Tooltip');
 goog.require('Blockly.Touch');
 goog.require('Blockly.utils');
+goog.require('Blockly.constants');
 
 goog.require('goog.Timer');
 goog.require('goog.asserts');
@@ -64,10 +65,25 @@ Blockly.BlockSvg = function(workspace, prototypeName, opt_id) {
    */
   this.svgGroup_ = Blockly.utils.createSvgElement('g', {}, null);
   /** @type {SVGElement} */
-  this.svgPath_ = Blockly.utils.createSvgElement('path',
-      {'class': 'blocklyPath blocklyBlockBackground'},
-      this.svgGroup_);
-  this.svgPath_.tooltip = this;
+  if (Blockly.theme === Blockly.Themes.CAT_BLOCKS) {
+    this.svgPath_ = Blockly.utils.createSvgElement('g', {}, this.svgGroup_);
+    this.svgPathBody_ = Blockly.utils.createSvgElement('path',
+        {'class': 'blocklyPath blocklyBlockBackground'}, this.svgPath_);
+
+    this.svgFace_ = Blockly.utils.createSvgElement('g', {},
+        this.svgPath_);
+    this.svgGroup_.svgPath = this.svgPath_;
+    this.svgPath_.svgFace = this.svgFace_;
+    this.svgPath_.svgBody = this.svgPathBody_;
+    this.lastCallTime = 0;
+    this.CALL_FREQUENCY_MS = 60;
+
+    this.svgPathBody_.tooltip = this;
+  } else {
+    this.svgPath_ = Blockly.utils.createSvgElement('path', {'class': 'blocklyPath blocklyBlockBackground'},
+        this.svgGroup_);
+    this.svgPath_.tooltip = this;
+  }
 
   /** @type {boolean} */
   this.rendered = false;
@@ -80,7 +96,7 @@ Blockly.BlockSvg = function(workspace, prototypeName, opt_id) {
    */
   this.useDragSurface_ = Blockly.utils.is3dSupported() && !!workspace.blockDragSurface_;
 
-  Blockly.Tooltip.bindMouseEvents(this.svgPath_);
+  Blockly.Tooltip.bindMouseEvents(this.blockFrameElement_);
   Blockly.BlockSvg.superClass_.constructor.call(this,
       workspace, prototypeName, opt_id);
 
@@ -160,6 +176,8 @@ Blockly.BlockSvg.prototype.initSvg = function() {
     for (i = 0; i < icons.length; i++) {
       icons[i].createIcon();
     }
+  } else if (this.svgPathBody_) {
+    this.svgPathBody_.setAttribute('stroke-opacity', '0');
   }
   this.updateColour();
   this.updateMovable();
@@ -174,6 +192,23 @@ Blockly.BlockSvg.prototype.initSvg = function() {
   }
 };
 
+Object.defineProperty(Blockly.BlockSvg.prototype, 'blockFrameElement_', {
+  /**
+   * The svg element (e.g. svgPath_ or svgPathBody_) that is
+   * responsible for the outline of the block, based on the current theme.
+   * @return {!SVGElement} The SVG element forming the outline.
+   * @this {Blockly.BlockSvg}
+   */
+  get: function() {
+    if (Blockly.theme === Blockly.Themes.CAT_BLOCKS) {
+      return this.svgPathBody_;
+    }
+    return this.svgPath_;
+  },
+  enumerable: true,
+  configurable: true
+});
+
 /**
  * Select this block.  Highlight it visually.
  */
@@ -218,6 +253,191 @@ Blockly.BlockSvg.prototype.unselect = function() {
   this.removeSelect();
 };
 
+Blockly.BlockSvg.prototype.initCatStuff = function() {
+  if (Blockly.theme !== Blockly.Themes.CAT_BLOCKS || this.hasInitCatStuff) {
+    return;
+  }
+  // TODO: Test what happens if we turn on and off Cat Blocks several times
+  this.hasInitCatStuff = true;
+
+  // Ear part of the SVG path for hat blocks
+  var LEFT_EAR_UP = 'c-1,-12.5 5.3,-23.3 8.4,-24.8c3.7,-1.8 16.5,13.1 18.4,15.4';
+  var LEFT_EAR_DOWN = 'c-5.8,-4.8 -8,-18 -4.9,-19.5c3.7,-1.8 24.5,11.1 31.7,10.1';
+  var RIGHT_EAR_UP = 'c1.9,-2.3 14.7,-17.2 18.4,-15.4c3.1,1.5 9.4,12.3 8.4,24.8';
+  var RIGHT_EAR_DOWN = 'c7.2,1 28,-11.9 31.7,-10.1c3.1,1.5 0.9,14.7 -4.9,19.5';
+  // Ears look slightly different for define hat blocks
+  var DEFINE_HAT_LEFT_EAR_UP = 'c0,-7.1 3.7,-13.3 9.3,-16.9c1.7,-7.5 5.4,-13.2 7.6,-14.2c2.6,-1.3 10,6 14.6,11.1';
+  var DEFINE_HAT_RIGHT_EAR_UP = 'h33c4.6,-5.1 11.9,-12.4 14.6,-11.1c1.9,0.9 4.9,5.2 6.8,11.1c2.6,0,5.2,0,7.8,0';
+  var DEFINE_HAT_LEFT_EAR_DOWN = 'c0,-4.6 1.6,-8.9 4.3,-12.3c-2.4,-5.6 -2.9,-12.4 -0.7,-13.4c2.1,-1 9.6,2.6 17,5.8' +
+    'c2.6,0 6.2,0 10.9,0';
+  var DEFINE_HAT_RIGHT_EAR_DOWN = 'c0,0 25.6,0 44,0c7.4,-3.2 14.8,-6.8 16.9,-5.8c1.2,0.6 1.6,2.9 1.3,5.8';
+
+  var that = this;
+  this.svgPath_.ear = Blockly.utils.createSvgElement('path', {}, this.svgPath_);
+  this.svgPath_.ear2 = Blockly.utils.createSvgElement('path', {}, this.svgPath_);
+  if (this.RTL) {
+    // Mirror the ears.
+    this.svgPath_.ear.setAttribute('transform', 'scale(-1 1)');
+    this.svgPath_.ear2.setAttribute('transform', 'scale(-1 1)');
+  }
+  this.svgPath_.addEventListener("mouseenter", function(event) {
+    clearTimeout(that.blinkFn);
+    // blink
+    if (event.target.svgFace.eye) {
+      event.target.svgFace.eye.setAttribute('fill-opacity','0');
+      event.target.svgFace.eye2.setAttribute('fill-opacity','0');
+      event.target.svgFace.closedEye.setAttribute('fill-opacity','0.6');
+      event.target.svgFace.closedEye2.setAttribute('fill-opacity','0.6');
+    }
+
+    // reset after a short delay
+    that.blinkFn = setTimeout(function() {
+      if (event.target.svgFace.eye) {
+        event.target.svgFace.eye.setAttribute('fill-opacity','0.6');
+        event.target.svgFace.eye2.setAttribute('fill-opacity','0.6');
+        event.target.svgFace.closedEye.setAttribute('fill-opacity','0');
+        event.target.svgFace.closedEye2.setAttribute('fill-opacity','0');
+      }
+    }, 100);
+  });
+
+  this.svgPath_.ear.addEventListener("mouseenter", function() {
+    clearTimeout(that.earFn);
+    clearTimeout(that.ear2Fn);
+    // ear flick
+    that.svgPath_.ear.setAttribute('fill-opacity','0');
+    that.svgPath_.ear2.setAttribute('fill-opacity','');
+    var bodyPath = that.svgPath_.svgBody.getAttribute('d');
+    bodyPath = bodyPath.replace(RIGHT_EAR_UP, RIGHT_EAR_DOWN);
+    bodyPath = bodyPath.replace(DEFINE_HAT_RIGHT_EAR_UP, DEFINE_HAT_RIGHT_EAR_DOWN);
+    bodyPath = bodyPath.replace(LEFT_EAR_DOWN, LEFT_EAR_UP);
+    bodyPath = bodyPath.replace(DEFINE_HAT_LEFT_EAR_DOWN, DEFINE_HAT_LEFT_EAR_UP);
+    that.svgPath_.svgBody.setAttribute('d', bodyPath);
+
+    // reset after a short delay
+    that.earFn = setTimeout(function() {
+      that.svgPath_.ear.setAttribute('fill-opacity','');
+      var bodyPath = that.svgPath_.svgBody.getAttribute('d');
+      bodyPath = bodyPath.replace(RIGHT_EAR_DOWN, RIGHT_EAR_UP);
+      bodyPath = bodyPath.replace(DEFINE_HAT_RIGHT_EAR_DOWN, DEFINE_HAT_RIGHT_EAR_UP);
+      that.svgPath_.svgBody.setAttribute('d', bodyPath);
+    }, 50);
+  });
+  this.svgPath_.ear2.addEventListener("mouseenter", function() {
+    clearTimeout(that.earFn);
+    clearTimeout(that.ear2Fn);
+    // ear flick
+    that.svgPath_.ear2.setAttribute('fill-opacity','0');
+    that.svgPath_.ear.setAttribute('fill-opacity','');
+    var bodyPath = that.svgPath_.svgBody.getAttribute('d');
+    bodyPath = bodyPath.replace(LEFT_EAR_UP, LEFT_EAR_DOWN);
+    bodyPath = bodyPath.replace(DEFINE_HAT_LEFT_EAR_UP, DEFINE_HAT_LEFT_EAR_DOWN);
+    bodyPath = bodyPath.replace(RIGHT_EAR_DOWN, RIGHT_EAR_UP);
+    bodyPath = bodyPath.replace(DEFINE_HAT_RIGHT_EAR_DOWN, DEFINE_HAT_RIGHT_EAR_UP);
+    that.svgPath_.svgBody.setAttribute('d', bodyPath);
+
+    // reset after a short delay
+    that.ear2Fn = setTimeout(function() {
+      that.svgPath_.ear2.setAttribute('fill-opacity','');
+      var bodyPath = that.svgPath_.svgBody.getAttribute('d');
+      bodyPath = bodyPath.replace(LEFT_EAR_DOWN, LEFT_EAR_UP);
+      bodyPath = bodyPath.replace(DEFINE_HAT_LEFT_EAR_DOWN, DEFINE_HAT_LEFT_EAR_UP);
+      that.svgPath_.svgBody.setAttribute('d', bodyPath);
+    }, 50);
+  });
+  this.windowListener = function(event) {
+    var time = Date.now();
+    if (time < that.lastCallTime + that.CALL_FREQUENCY_MS) return;
+    that.lastCallTime = time;
+    if (!that.shouldWatchMouse()) return;
+
+    // mouse watching
+    if (that.workspace) { // not disposed
+      var xy = that.getCatFacePosition();
+      var mouseLocation = {
+        x: event.x / that.workspace.scale,
+        y: event.y / that.workspace.scale
+      };
+
+      var dx = mouseLocation.x - xy.x;
+      var dy = mouseLocation.y - xy.y;
+      var theta = Math.atan2(dx, dy);
+
+      // Map the vector from the cat face to the mouse location to a much shorter
+      // vector in the same direction, which will be the translation vector for
+      // the cat face
+      var delta = Math.sqrt(dx * dx + dy * dy);
+      var scaleFactor = delta / (delta + 1);
+
+      // Equation for radius of ellipse at theta for axes with length a and b
+      var a = 2;
+      var b = 5;
+      var r = a * b / Math.sqrt(Math.pow(b * Math.cos(theta), 2) + Math.pow(a * Math.sin(theta), 2));
+
+      // Convert polar coordinate back to x, y coordinate
+      dx = (r * scaleFactor) * Math.sin(theta);
+      dy = (r * scaleFactor) * Math.cos(theta);
+
+      if (that.RTL) dx -= 87; // Translate face over
+      that.svgFace_.style.transform = 'translate(' + dx + 'px, ' + dy + 'px)';
+    }
+  };
+  if (this.RTL) {
+    // Set to the correct initial position
+    this.svgFace_.style.transform = 'translate(-87px, 0px)';
+  }
+  if (this.shouldWatchMouse()) {
+    document.addEventListener('mousemove', this.windowListener);
+  }
+};
+
+/**
+ * Get cat face position
+ * @return {Object} coordinates of center of cat face
+ */
+Blockly.BlockSvg.prototype.getCatFacePosition = function() {
+  // getBoundingClientRect is not performant
+  //var offset = that.workspace.getParentSvg().getBoundingClientRect();
+  var offset = {x:0, y:92};
+
+  offset.x += 120; // scratchCategoryMenu width
+
+  if (!this.isInFlyout && this.workspace.getFlyout()) {
+    offset.x += this.workspace.getFlyout().getWidth();
+  }
+
+  offset.x += this.workspace.scrollX;
+  offset.y += this.workspace.scrollY;
+
+  var xy = this.getRelativeToSurfaceXY(this.svgGroup_);
+  if (this.RTL) {
+    xy.x = this.workspace.getWidth() - xy.x - this.width;
+  }
+  // convert to workspace units
+  xy.x += offset.x / this.workspace.scale;
+  xy.y += offset.y / this.workspace.scale;
+  // distance to center of face
+  xy.x -= 43.5;
+  xy.y -= 4;
+  if (this.RTL) {
+    // We've been calculating from the right edge. Convert x to from left edge.
+    xy.x = screen.width - xy.x;
+  }
+  return xy;
+};
+
+/**
+ * True if cat should watch mouse
+ * @return {boolean} true if the block should be watching the mouse
+ */
+Blockly.BlockSvg.prototype.shouldWatchMouse = function() {
+  if (window.vmLoadHigh || !window.CAT_CHASE_MOUSE) return false;
+  var xy = this.getCatFacePosition();
+  var blockXOnScreen = xy.x > 0 && xy.x < screen.width / this.workspace.scale;
+  var blockYOnScreen = xy.y > 0 && xy.y < screen.height / this.workspace.scale;
+  return this.startHat_ && !this.isGlowingStack_ && blockXOnScreen && blockYOnScreen;
+};
+
 /**
  * Glow only this particular block, to highlight it visually as if it's running.
  * @param {boolean} isGlowingBlock Whether the block should glow.
@@ -232,6 +452,23 @@ Blockly.BlockSvg.prototype.setGlowBlock = function(isGlowingBlock) {
  * @param {boolean} isGlowingStack Whether the stack starting with this block should glow.
  */
 Blockly.BlockSvg.prototype.setGlowStack = function(isGlowingStack) {
+  if (Blockly.theme === Blockly.Themes.CAT_BLOCKS) {
+    if (isGlowingStack) {
+      // For performance, don't follow the mouse when the stack is glowing
+      document.removeEventListener('mousemove', this.windowListener);
+      if (this.workspace && this.svgFace_.style) {
+        // reset face direction
+        if (this.RTL) {
+          this.svgFace_.style.transform = 'translate(-87px, 0px)';
+        } else {
+          this.svgFace_.style.transform = '';
+        }
+      }
+    } else {
+      document.addEventListener('mousemove', this.windowListener);
+    }
+  }
+
   this.isGlowingStack_ = isGlowingStack;
   // Update the applied SVG filter if the property has changed
   var svg = this.getSvgRoot();
@@ -822,6 +1059,18 @@ Blockly.BlockSvg.prototype.dispose = function(healStack, animate) {
     // The block has already been deleted.
     return;
   }
+  if (this.blinkFn) {
+    clearTimeout(this.blinkFn);
+  }
+  if (this.earFn) {
+    clearTimeout(this.earFn);
+  }
+  if (this.ear2Fn) {
+    clearTimeout(this.ear2Fn);
+  }
+  if (this.windowListener) {
+    document.removeEventListener('mousemove', this.windowListener);
+  }
   Blockly.Tooltip.hide();
   Blockly.Field.startCache();
   // Save the block's workspace temporarily so we can resize the
@@ -860,6 +1109,8 @@ Blockly.BlockSvg.prototype.dispose = function(healStack, animate) {
   // Sever JavaScript to DOM connections.
   this.svgGroup_ = null;
   this.svgPath_ = null;
+  this.svgPathBody_ = null;
+  this.svgFace_ = null;
   Blockly.Field.stopCache();
 };
 
diff --git a/core/blockly.js b/core/blockly.js
index fa0b484896..0996503923 100644
--- a/core/blockly.js
+++ b/core/blockly.js
@@ -112,6 +112,13 @@ Blockly.clipboardSource_ = null;
  */
 Blockly.cache3dSupported_ = null;
 
+/**
+ * Active theme.
+ * @type {!Blockly.Themes}
+ * @private
+ */
+Blockly.theme_ = Blockly.Themes.CLASSIC;
+
 /**
  * Convert a hue (HSV model) into an RGB hex triplet.
  * @param {number} hue Hue on a colour wheel (0-360).
@@ -175,6 +182,28 @@ Blockly.svgResize = function(workspace) {
   mainWorkspace.resize();
 };
 
+/**
+ * Apply a global theme to Blockly. This will then be used in all workspaces -
+ * both newly created and already existing.
+ */
+Object.defineProperty(Blockly, 'theme', {
+  /**
+   * Get the current theme.
+   * @return {!Blockly.Themes} The current global theme.
+   */
+  get: function() {
+    return Blockly.theme_;
+  }
+});
+
+Blockly.setTheme = function(theme) {
+  if (theme === Blockly.Themes.CAT_BLOCKS) {
+    Blockly.theme_ = theme;
+  } else {
+    Blockly.theme_ = Blockly.Themes.CLASSIC;
+  }
+};
+
 /**
  * Handle a key-down on SVG drawing surface. Does nothing if the main workspace is not visible.
  * @param {!Event} e Key down event.
diff --git a/core/constants.js b/core/constants.js
index 2b6d653586..539879c164 100644
--- a/core/constants.js
+++ b/core/constants.js
@@ -385,3 +385,12 @@ Blockly.StatusButtonState = {
   "READY": "ready",
   "NOT_READY": "not ready",
 };
+
+/**
+ * ENUM defining supported themes.
+ * @enum {string}
+ */
+Blockly.Themes = {
+  CLASSIC: "classic",
+  CAT_BLOCKS: "catblocks"
+};