Implement theme admin UI to support previewing

LEARNER-2017

themes/red-theme/cms/static/sass/partials/base/_variables.scss

@@ -1,14 +0,0 @@
-// Color overrides
-$white: rgb(255,255,255);
-$red: #d9534f !default;
-
-$footer-bg: $white;
-$header-bg: $white;
-$header-border-color: $red;
-
-$base-font-color: $red;
-$link-color: $red;
-$lms-active-color: $red;
-$lms-label-color: $red;
-
-@import 'lms/static/sass/partials/base/variables';

themes/red-theme/cms/static/sass/partials/cms/bootstrap/_theme.scss

@@ -3,7 +3,7 @@
 // Theme colors
 //
 // Note: define colors needed by your theme first
-$red: #d9534f !default;
+$red: #d9534f;
 $brand-primary: $red;
 
 // Theme fonts