Local preview: Ensure background blur is opaque to items behind it

stylesheets/_modules.scss

@@ -4551,6 +4551,10 @@ button.module-image__border-overlay:focus {
       bottom: 16px;
     }
 
+    &__background {
+      background-color: variables.$color-gray-78;
+    }
+
     &__video {
       height: 100%;
       width: 100%;

ts/components/CallScreen.tsx

@@ -508,7 +508,10 @@ export function CallScreen({
         ref={setLocalPreviewContainer}
       />
     ) : (
-      <CallBackgroundBlur avatarUrl={me.avatarUrl}>
+      <CallBackgroundBlur
+        className="module-ongoing-call__local-preview__background"
+        avatarUrl={me.avatarUrl}
+      >
         <Avatar
           avatarPlaceholderGradient={me.avatarPlaceholderGradient}
           avatarUrl={me.avatarUrl}