Game Modes can be controlled via the Admin -> More Admin -> Game Modes menu.
First create the mode in the admin menu. The mode will then appear in the play match selector.
Game modes can be set up with different rules than normal. In game mode specific rule cards, input a custom card property reqGameMode
with the id of the game mode:
Game mode rule cards will always load after all the other rules cards, so they can overwrite properties previously set by the normal rules loaded earlier.