adjust styles for login page

resource/styles/scss/_login.scss

@@ -226,6 +226,7 @@
       // 'backface-visibility: hidden' and 'z-index: -1' breaks layout in iOS
       .fcbtn:after {
         z-index: 0;
+        opacity: 0.3;
       }
     }
     &.to-flip .back {