fix(menu): support dark themes and non-grey backgrounds

Menu popups should always be lightly colored and its content
should use their respective contrast color and opactiy.

 - Set background color to background-50
 - Use contrast values for buttons, icons, and dividers

Fixes #9100

Author: clshortfuse

src/components/menu/menu-theme.scss

@@ -1,24 +1,18 @@
 md-menu-content.md-THEME_NAME-theme {
-  background-color: '{{background-A100}}';
-
+  background-color: '{{background-50}}';
   md-menu-item {
-    color: '{{background-A200-0.87}}';
-
+    color: '{{background-50-contrast}}';
     md-icon {
-      color: '{{background-A200-0.54}}';
+      color: '{{background-50-contrast-icon}}';
     }
-
     .md-button[disabled] {
-      color: '{{background-A200-0.25}}';
-
+      color: '{{background-50-contrast-disabled}}';
       md-icon {
-        color: '{{background-A200-0.25}}';
+        color: '{{background-50-contrast-disabled}}';
       }
     }
-
   }
-
   md-menu-divider {
-    background-color: '{{background-A200-0.11}}';
+    background-color: '{{background-50-contrast-divider}}';
   }
 }