[Glitch] Reset background color on media modal

Port 882afd7748 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/features/ui/components/media_modal.tsx

@@ -116,6 +116,9 @@ export const MediaModal: FC<MediaModalProps> = forwardRef<
           onChangeBackgroundColor(backgroundColor);
         }
       }
+      return () => {
+        onChangeBackgroundColor(null);
+      };
     }, [currentMedia, onChangeBackgroundColor]);
 
     const [viewportDimensions, setViewportDimensions] = useState<{