Add tooltip to theme-picker

Author: jelbourn

src/app/shared/theme-chooser/theme-chooser.html

@@ -1,4 +1,4 @@
-<button md-icon-button [md-menu-trigger-for]="themeMenu">
+<button md-icon-button [md-menu-trigger-for]="themeMenu" mdTooltip="Select a theme!">
   <md-icon>format_color_fill</md-icon>
 </button>
 

src/app/shared/theme-chooser/theme-chooser.ts

@@ -9,7 +9,7 @@ import {ThemeStorage, DocsSiteTheme} from './theme-storage/theme-storage';
   styleUrls: ['theme-chooser.scss'],
   changeDetection: ChangeDetectionStrategy.OnPush,
   encapsulation: ViewEncapsulation.None,
-  host: { 'aria-hidden': 'true' },
+  host: {'aria-hidden': 'true'},
 })
 export class ThemeChooser {
   currentTheme;