feat(card): add default background color

Closes #1846

Author: Marcy Sutton

src/components/card/card-theme.scss

@@ -1,6 +1,7 @@
 $card-border-radius: 2px !default;
 
 md-card.md-THEME_NAME-theme {
+  background-color: '{{background-hue-3}}';
   border-radius: $card-border-radius;
 
   .md-card-image {