Bug 776726 followup: use background, not background-color, to override default styling, r=oops

--HG--
extra : transplant_source : e%C38%C2Gx%D5/%89%C2%B6%F5%11jU.%5B%93%26%CE

browser/themes/pinstripe/browser.css

@@ -3459,7 +3459,7 @@ stack[anonid=browserStack][responsivemode] {
   margin: -4px 0 0 0;
   padding: 0;
   border-radius: 0px;
-  background-color: white;
+  background: white;
 }
 
 #social-notification-panel .panel-arrow[side="top"] {