Ensure arrow doesn't change on disabled <select>

We can't just modify the CSS variable here, since that is also used in
the style for :disabled. We need to change the entire "background-image"
in order for :disabled to be able to override it.

app/styles/input.css

@@ -68,13 +68,13 @@ select {
           https://bugzilla.mozilla.org/show_bug.cgi?id=1805406 */
 select:active {
   /* Rotated arrow */
-  --select-arrow: url('data:image/svg+xml;utf8, \
+  background-image: url('data:image/svg+xml;utf8, \
       <svg width="8" height="6" version="1.1" viewBox="0 0 8 6" \
            xmlns="http://www.w3.org/2000/svg" transform="rotate(180)" > \
           <path d="m6.5 1.5 -2.5 3 -2.5 -3 5 0" stroke-width="3" \
                 stroke="rgb(31,31,31)" fill="none" \
                 stroke-linecap="round" stroke-linejoin="round" /> \
-      </svg>');
+      </svg>'), var(--bg-gradient);
 }
 option {
   color: black;