Add background to media modal (#4317)

app/javascript/styles/components.scss

@@ -3057,6 +3057,10 @@ button.icon-button.active i.fa-retweet {
   }
 }
 
+.media-modal__content {
+  background: $base-overlay-background;
+}
+
 .media-modal__close {
   position: absolute;
   right: 4px;