fix(theming): add support for background hues

Closes: #1450, closes #1950, closes #1321

Author: arshabh

src/components/content/content-theme.scss

@@ -1,3 +1,3 @@
 md-content.md-THEME_NAME-theme {
-  background-color: '{{background-hue-3}}';
+  background-color: '{{background-color}}';
 }

src/components/dialog/dialog-theme.scss

@@ -2,7 +2,7 @@ $dialog-border-radius: 4px !default;
 
 md-dialog.md-THEME_NAME-theme {
   border-radius: $dialog-border-radius;
-  background-color: '{{background-hue-3}}';
+  background-color: '{{background-color}}';
 
   &.md-content-overflow .md-actions {
     border-top-color: '{{foreground-4}}';

src/components/sidenav/sidenav-theme.scss

@@ -1,3 +1,3 @@
 md-sidenav.md-THEME_NAME-theme {
-  background-color: '{{background-hue-3}}';
+  background-color: '{{background-color}}';
 }

src/components/slider/slider-theme.scss

@@ -13,10 +13,10 @@ md-slider.md-THEME_NAME-theme {
     border-color: '{{foreground-4}}';
   }
   .md-disabled-thumb {
-    border-color: '{{background-hue-3}}';
+    border-color: '{{background-color}}';
   }
   &.md-min .md-thumb:after {
-    background-color: '{{background-hue-3}}';
+    background-color: '{{background-color}}';
   }
 
   .md-track.md-track-fill {

src/components/subheader/subheader-theme.scss

@@ -1,6 +1,6 @@
 .md-subheader.md-THEME_NAME-theme {
   color: '{{ foreground-2-0.23 }}';
-  background-color: '{{background-hue-3}}';
+  background-color: '{{background-color}}';
 
   &.md-primary {
     color: '{{primary-color}}'