[BS5] remove ui theme selector - add darkmode toggler

data/web/lang/lang.en.json

@@ -338,7 +338,6 @@
         "ui_header_announcement_type_info": "Info",
         "ui_header_announcement_type_warning": "Important",
         "ui_texts": "UI labels and texts",
-        "ui_theme": "Design",
         "unban_pending": "unban pending",
         "unchanged_if_empty": "If unchanged leave blank",
         "upload": "Upload",