DOC: css for code blocks

Author: gsmolk

doc/stylesheet.css

@@ -400,3 +400,10 @@ table.CAUTION,
     background: #FFF;
     width: auto;
 }
+
+.book pre {
+    background: #f5f5f5;
+    padding: 10px;
+    border: 1px solid #ccc;
+    border-radius: 4px;
+}