Add an override for the theme

So we can force the light theme on unthemeable pages like the login
& complete security page.

Fixes https://github.com/vector-im/riot-web/issues/12149
Fixes https://github.com/vector-im/riot-web/issues/12157

src/settings/controllers/ThemeController.js

@@ -19,9 +19,17 @@ import SettingController from "./SettingController";
 import {DEFAULT_THEME, enumerateThemes} from "../../theme";
 
 export default class ThemeController extends SettingController {
+    static isLogin = false;
+
+    static setIsLogin(val) {
+        ThemeController.isLogin = val;
+    }
+
     getValueOverride(level, roomId, calculatedValue, calculatedAtLevel) {
         if (!calculatedValue) return null; // Don't override null themes
 
+        if (ThemeController.isLogin) return 'light';
+
         const themes = enumerateThemes();
         // Override in case some no longer supported theme is stored here
         if (!themes[calculatedValue]) {