fix(bottom-sheet): component theme now supports dark mode (#11380)

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## PR Checklist
Please check that your PR fulfills the following requirements:
- [x] The commit message follows [our guidelines](https://github.com/angular/material/blob/master/.github/CONTRIBUTING.md#-commit-message-format)
- [ ] Tests for the changes have been added or this is not a bug fix / enhancement
- [x] Docs have been added, updated, or were not required

## PR Type
What kind of change does this PR introduce?
<!-- Please check the one that applies to this PR using "x". -->
```
[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:
```

## What is the current behavior?
See Before screenshots below.
<!-- Please describe the current behavior that you are modifying and link to one or more relevant issues. -->
Issue Number: 
N/A

## What is the new behavior?
See After screenshots below.

## Does this PR introduce a breaking change?
```
[ ] Yes
[x] No
```
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## Other information

### Before
![screen shot 2018-07-22 at 4 11 13 am](https://user-images.githubusercontent.com/3506071/43043610-6d51e650-8d65-11e8-8c2a-da73944f872e.png)
![screen shot 2018-07-22 at 4 11 24 am](https://user-images.githubusercontent.com/3506071/43043611-6d5e7b9a-8d65-11e8-91c5-57c58bf606e3.png)
![screen shot 2018-07-22 at 4 11 48 am](https://user-images.githubusercontent.com/3506071/43043612-6d6d5728-8d65-11e8-96e0-010c4d1cd176.png)
![screen shot 2018-07-22 at 4 11 55 am](https://user-images.githubusercontent.com/3506071/43043613-6d7c034a-8d65-11e8-9577-95228b822c95.png)

### After
![screen shot 2018-07-22 at 4 07 50 am](https://user-images.githubusercontent.com/3506071/43043600-3475240a-8d65-11e8-968d-a1dd23475a74.png)
![screen shot 2018-07-22 at 4 08 04 am](https://user-images.githubusercontent.com/3506071/43043601-348382fc-8d65-11e8-91d4-cb9ce955bed3.png)
![screen shot 2018-07-22 at 4 09 29 am](https://user-images.githubusercontent.com/3506071/43043602-34922528-8d65-11e8-997a-343ad874073f.png)
![screen shot 2018-07-22 at 4 09 38 am](https://user-images.githubusercontent.com/3506071/43043603-349ebf40-8d65-11e8-8cb9-e797d618756b.png)

Author: Splaktar

src/components/bottomSheet/bottom-sheet-theme.scss

@@ -1,7 +1,6 @@
-
 md-bottom-sheet.md-THEME_NAME-theme {
-  background-color: '{{background-50}}';
-  border-top-color: '{{background-300}}';
+  background-color: '{{background-color}}';
+  border-top-color: '{{background-hue-3}}';
 
   &.md-list {
     md-list-item {
@@ -10,7 +9,7 @@ md-bottom-sheet.md-THEME_NAME-theme {
   }
 
   .md-subheader {
-    background-color: '{{background-50}}';
+    background-color: '{{background-color}}';
   }
 
   .md-subheader {

src/components/bottomSheet/demoBasicUsage/bottom-sheet-list-template.html

@@ -6,7 +6,7 @@
       <md-button
           ng-click="listItemClick($index)"
           md-autofocus="$index == 2"
-          class="md-list-item-content" >
+          class="md-list-item-content">
         <md-icon md-svg-src="{{item.icon}}"></md-icon>
         <span class="md-inline-list-icon-label">{{ item.name }}</span>
       </md-button>

src/components/bottomSheet/demoBasicUsage/style.css

@@ -1,47 +1,35 @@
 .md-inline-list-icon-label {
   display: inline-block;
   padding-left: 10px;
-  padding-right:10px;
+  padding-right: 10px;
   margin-top: -10px;
   height: 24px;
   vertical-align: middle;
-
 }
-
 .md-grid-item-content {
-  height:90px;
-  padding-top:10px;
+  height: 90px;
+  padding-top: 10px;
 }
 .md-grid-item-content md-icon {
   height: 48px;
   width: 48px;
 }
-
 .md-grid-text {
   padding-bottom: 5px;
-  }
-
-
+}
 md-list-item, md-list-item .md-list-item-inner {
   min-height: 48px;
-
 }
-
 h2 {
   line-height: 36px;
-    padding-top: 10px;
+  padding-top: 10px;
 }
-
-
 .md-subheader .md-subheader-inner {
   padding: 0;
 }
-
-
 md-toast .md-toast-content {
   background-color: #B14141;
 }
-
 md-toast > * {
   font-weight: bolder;
 }