Fix @keyframes.

Use the `@at-root` directive to work around the issue.

sass/theme-ios.scss

@@ -580,7 +580,9 @@ textarea,
 }
 
 // Overlay animation for the slide transition
+@at-root {
   @-webkit-keyframes fadeOverlay {
     from { @include box-shadow(0 0 10px rgba(0,0,0,0), -320px 0 0 rgba(0,0,0,0)); }
     to   { @include box-shadow(0 0 10px rgba(0,0,0,.3), -320px 0 0 rgba(0,0,0,.1)); }
   }
+}