use pretty throbber for log in via Developer Hub (bug 792599)

media/css/devreg/header.less

@@ -14,15 +14,16 @@
     }
     /* Log in link loader */
     a.loading-submit:before {
-        background-image: url(../../img/zamboni/loading-white.gif);
+        background-image: url(../../img/zamboni/mobile/loading-white.png);
+        background-size: auto 16px;
         content: "";
         display: block;
         height: 16px;
-        left: -20px;
-        margin-right: 10px;
-        margin-top: -8px;
+        margin-right: 5px;
+        margin-top: -7px;
         position: absolute;
         top: 50%;
+        left: -14px;
         width: 16px;
     }
 }