fix(sidenav): fix dark theme (#1319)

Author: kara

src/lib/sidenav/_sidenav-theme.scss

@@ -14,10 +14,17 @@
   // we use a light backdrop.
   $sidenav-backdrop-color: invert(md-color($background, card, 0.6)) !default;
   $sidenav-background-color: md-color($background, dialog) !default;
+  $sidenav-layout-background-color:  md-color($background, background) !default;
   $sidenav-push-background-color: md-color($background, dialog) !default;
 
+  md-sidenav-layout {
+    background-color: $sidenav-layout-background-color;
+    color: md-color($foreground, text);
+  }
+
   md-sidenav {
     background-color: $sidenav-background-color;
+    color: md-color($foreground, text);
 
     &.md-sidenav-push {
       background-color: $sidenav-push-background-color;