Add muted style for polls in web UI (#10248)

app/javascript/styles/mastodon/polls.scss

@@ -190,3 +190,15 @@
     color: darken($simple-background-color, 14%);
   }
 }
+
+.muted .poll {
+  color: $dark-text-color;
+
+  &__chart {
+    background: rgba(darken($ui-primary-color, 14%), 0.2);
+
+    &.leading {
+      background: rgba($ui-highlight-color, 0.2);
+    }
+  }
+}