What part in particular are you stuck on?
You'll want to
1. Add a new variable to the _settings struct in game_settings for the new option.
2. Add the name of the new option to match_settings_choices in obj_css_ui: Room Start.
3. Change the Step and Draw End events of obj_css_ui to handle the new option.
Once you've added all the rules to the menu, you can decide where to put the code that checks the match settings and applies the rule. That'll probably be in game_object_setup.
