Increase contrast of upload progress background (#23836)

app/javascript/styles/mastodon-light/diff.scss

@@ -258,6 +258,10 @@ html {
   border-color: $ui-base-color;
 }
 
+.upload-progress__backdrop {
+  background: $ui-base-color;
+}
+
 // Change the background colors of statuses
 .focusable:focus {
   background: $ui-base-color;