[Catalyst] Migrate "Dark theme" settings

Bug: 375132235
Flag: com.android.settings.flags.catalyst_dark_ui_mode
Test: Manual
Change-Id: I40f305fe8d89feac84e2316c7a90c6eeb2b4a5fa
diff --git a/aconfig/catalyst/display.aconfig b/aconfig/catalyst/display.aconfig
index 94a01e0..038a9b0 100644
--- a/aconfig/catalyst/display.aconfig
+++ b/aconfig/catalyst/display.aconfig
@@ -14,3 +14,11 @@
   description: "Flag for Screen Timeout settings"
   bug: "323791114"
 }
+
+flag {
+  name: "catalyst_dark_ui_mode"
+  namespace: "android_settings"
+  description: "Flag for Dark theme"
+  bug: "323791114"
+}
+