[Web] Update Bootstrap to 5.3.8, add theme support

data/web/admin/system.php

@@ -115,6 +115,8 @@ $template_data = [
   'logo_dark_specs' => customize('get', 'main_logo_dark_specs'),
   'ip_check' => customize('get', 'ip_check'),
   'custom_login' => customize('get', 'custom_login'),
+  'ui_themes' => customize('get', 'ui_themes'),
+  'ui_theme' => customize('get', 'ui_theme'),
   'password_complexity' => password_complexity('get'),
   'show_rspamd_global_filters' => @$_SESSION['show_rspamd_global_filters'],
   'cors_settings' => $cors_settings,