You’re going to have to edit the default chat resource, which is not recommended. But if you really, really want to change the chat resource you should do the following:
In index.css change this (line 79):
background-color: rgba(44, 62, 80, 1.0);
background-color: rgba(44, 62, 80, 0.0);
You could cancel all chat message events unless the message was sent using /ooc or /tweet.