Ensure the settings page accurately represents theme choices

Fixes https://github.com/vector-im/riot-web/issues/11518

This also fixes a bug where the the theme logic can incorrectly get stuck in the light theme - the ThemeController was overriding *all* values, not just supposed themes. Null values aren't overridden so that the various theme logic bits don't assume the user has chosen the light theme explicitly.

src/settings/controllers/ThemeController.js

@@ -20,6 +20,8 @@ import {DEFAULT_THEME, enumerateThemes} from "../../theme";
 
 export default class ThemeController extends SettingController {
     getValueOverride(level, roomId, calculatedValue, calculatedAtLevel) {
+        if (!calculatedValue) return null; // Don't override null themes
+
         const themes = enumerateThemes();
         // Override in case some no longer supported theme is stored here
         if (!themes[calculatedValue]) {