Refine PS1/PS2/PS3/NDS themes and add decorative background field

Rework console theme stylesheets and add opt-in .rb-bg cube field
(used by PS2 BIOS bobbing-cube background) wired through Shell.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/globals.css

@@ -50,6 +50,11 @@ img {
   padding: 0 0 40px; /* room for the fixed taskbar */
 }
 
+/* Opt-in decorative background field (themes enable it). */
+.rb-bg {
+  display: none;
+}
+
 .rb-window {
   flex: 1;
   display: flex;