Flarum\Admin\AdminServiceProvider::flushWebAppAssetsWhenThemeChanged PHP Method

    protected function flushWebAppAssetsWhenThemeChanged()
    {
        $this->app->make('events')->listen(SettingWasSet::class, function (SettingWasSet $event) {
            if (preg_match('/^theme_|^custom_less$/i', $event->key)) {
                $this->getWebAppAssets()->flushCss();
            }
        });
    }