[Glitch] Fix audio player modal having white-on-white buttons in light theme

Port 02fe176fd4 to glitch-soc

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

app/javascript/flavours/glitch/features/ui/components/video_modal.jsx

@@ -37,6 +37,10 @@ class VideoModal extends ImmutablePureComponent {
     }
   }
 
+  componentWillUnmount () {
+    this.props.onChangeBackgroundColor(null);
+  }
+
   render () {
     const { media, status, onClose } = this.props;
     const options = this.props.options || {};