Fix bg card for zwa2 install

Author: stvncode

home-assistant-connect-zwa-2/install.html

@@ -34,7 +34,7 @@
       }
 
       .installer-card {
-        background: white;
+        background: var(--app-bg-card);
         border-radius: 12px;
         padding: 20px;
         box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);