Make explicit storage item preserve/remove behavior on unlink

ts/util/theme.std.ts

@@ -1,6 +1,7 @@
 // Copyright 2020 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
+import { z } from 'zod';
 import { missingCaseError } from './missingCaseError.std.js';
 import { ThemeType } from '../types/Util.std.js';
 
@@ -9,6 +10,9 @@ export enum Theme {
   Dark,
 }
 
+export const themeSettingSchema = z.enum(['system', 'light', 'dark']);
+export type ThemeSettingType = z.infer<typeof themeSettingSchema>;
+
 export function themeClassName(theme: Theme): string {
   switch (theme) {
     case Theme.Light: