fix: Fix selected item in poll select menus is unreadable in Firefox (#35402)

app/javascript/styles/mastodon/components.scss

@@ -939,6 +939,7 @@ body > [data-popper-placement] {
         line-height: 20px;
         letter-spacing: 0.1px;
         color: $highlight-text-color;
+        background-color: var(--input-background-color);
         white-space: nowrap;
         text-overflow: ellipsis;
         overflow: hidden;