enhance(frontend): テーマの適用管理を改善 (#17376)

* wip

* add test

* use themeManager.currentCompiledTheme for obtaining theme variables / reduce getComputedStyle usage

* fix

* fix: better error handling on theme installation

* Update Changelog

* chore: remove frontend-shared builds as it is currently working as a stub package

* fix: broken lockfile

* fix

* fix lint

* fix

packages/i18n/src/autogen/locale.ts

@@ -5651,6 +5651,14 @@ export interface Locale extends ILocale {
      * リノート先のチャンネルを見る
      */
     "viewRenotedChannel": string;
+    /**
+     * テーマのプレビュー中
+     */
+    "previewingTheme": string;
+    /**
+     * 元に戻す
+     */
+    "previewingThemeRestore": string;
     "_imageEditing": {
         "_vars": {
             /**