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;

app/javascript/styles/mastodon/components.scss

@@ -4543,7 +4543,7 @@ a.status-card.compact:hover {
   width: 100%;
   height: 6px;
   border-radius: 6px;
-  background: $ui-base-lighter-color;
+  background: darken($simple-background-color, 8%);
   position: relative;
   margin-top: 5px;
 }