Re-implement UI reskin with working legacy toggle

Re-implement UI reskin with working legacy toggle; add return to title option to menu

src/enums/battle-spec.ts

@@ -1,4 +1,3 @@
-
 export enum BattleSpec {
     DEFAULT,
     FINAL_BOSS

src/enums/ui-theme.ts

@@ -0,0 +1,4 @@
+export enum UiTheme {
+  DEFAULT,
+  LEGACY
+}