Properly cache blur effect

res/css/structures/_BackdropPanel.scss

@@ -20,6 +20,7 @@ limitations under the License.
   top: 0;
   height: 100vh;
   width: 100%;
+  overflow: hidden;
 
   &:before {
     content: ' ';

res/themes/dark/css/dark.scss

@@ -2,10 +2,6 @@
 @import "../../light/css/_paths.scss";
 @import "../../light/css/_fonts.scss";
 @import "../../light/css/_light.scss";
-// important this goes before _mods,
-// as $groupFilterPanel-background-blur-amount and
-// $roomlist-background-blur-amount
-// are overridden in _dark.scss
 @import "_dark.scss";
 @import "../../light/css/_mods.scss";
 @import "../../../../res/css/_components.scss";