Update backgrounds.css

Adding in a bg-none helper class (we need it for the way we've chosen to do a link background instead of an underline).

Author: MelSumner

addon/styles/backgrounds.css

@@ -65,6 +65,10 @@
   background: var(--color-info);
 }
 
+.bg-none {
+  background: none;
+}
+
 .text-light {
   color: var(--color-white);