Improve the contrast of the chart in “poll has ended” notifications (#22575)

* Improve the contrast of the chart in “poll has ended” notifications

* Further increase the contrast of non-leading poll options

app/javascript/styles/mastodon/polls.scss

@@ -279,10 +279,10 @@
   color: $dark-text-color;
 
   &__chart {
-    background: rgba(darken($ui-primary-color, 14%), 0.2);
+    background: rgba(darken($ui-primary-color, 14%), 0.7);
 
     &.leading {
-      background: rgba($ui-highlight-color, 0.2);
+      background: rgba($ui-highlight-color, 0.5);
     }
   }
 }