Set currently selected key's color to the primary color. #21

This commit relates to #18 as well, as we will have to refactor the
color set here after finishing that issue.

sass/app.scss

@@ -183,18 +183,10 @@ ul {
 }
 
 svg.uhk {
-  &.faded {
+  rect {
-    #left-case,
+    &.active {
-    #right-case {
+      //TODO: use a color variable instead of the declaration below.
-      fill: #555;
+      fill: #337ab7;
-    }
-
-    rect {
-      fill: #666;
-
-      &.active {
-        fill: #000;
-      }
     }
   }
 }