Add checkmark symbol to checkbox (#22795)

app/javascript/styles/mastodon/components.scss

@@ -423,7 +423,7 @@ body > [data-popper-placement] {
 
       &.active {
         border-color: $highlight-text-color;
-        background: $highlight-text-color;
+        background: $highlight-text-color url("data:image/svg+xml;utf8,<svg width='18' height='18' fill='none' xmlns='http://www.w3.org/2000/svg'><path d='M4.5 8.5L8 12l6-6' stroke='white' stroke-width='1.5'/></svg>") center center no-repeat;
       }
     }
   }