Fix visibility of completed checkmarks on onyx theme

Ensure checkmarks under the "completed" state are visible on the
near-white primary background for the onyx theme. The change adjusts
selector scoping so dark text and icons (including elements with
.text-white inside .bg-primary) are explicitly styled with a dark color,
preventing white-on-near-white contrast issues.

apps/tauri/src/app.css

@@ -166,8 +166,9 @@ body {
   color: #d1bf82;
 }
 
-/* Dark text on primary buttons/FAB */
-[data-theme="onyx"] .bg-primary {
+/* Dark text/icons on primary buttons/FAB */
+[data-theme="onyx"] .bg-primary,
+[data-theme="onyx"] .bg-primary .text-white {
   color: #141414;
 }