Dark mode: dropdown arrow no longer tiles across selects

The dark .form-control override used the background shorthand, which
resets a select's background-repeat and position; the dark select rule
then re-added the arrow image without them, tiling it from the top
left. Use background-color in the overrides and restate
no-repeat/position on the select rule.

frontend/src/assets/style.css

@@ -628,18 +628,22 @@ input[type="radio"] {
    theme store always stamps it at startup) - a bare prefers-color-scheme
    query here leaks dark widget styles into light mode on dark-OS machines. */
 [data-theme="dark"] .form-control {
-  background: var(--bg);
+  /* background-COLOR, not the shorthand: the shorthand resets a select's
+     background-repeat/position and the dropdown arrow tiles across the box. */
+  background-color: var(--bg);
   border-color: var(--border);
 }
 
 [data-theme="dark"] .form-control:focus {
-  background: var(--bg);
+  background-color: var(--bg);
   box-shadow: 0 0 0 3px rgba(96, 165, 250, 0.2);
 }
 
 [data-theme="dark"] select.form-control {
   background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' viewBox='0 0 12 12'%3E%3Cpath fill='%23999' d='M6 8L1 3h10z'/%3E%3C/svg%3E");
   background-color: var(--bg);
+  background-repeat: no-repeat;
+  background-position: right 0.75rem center;
 }
 
 [data-theme="dark"] select.form-control option {