If your chat is on the Community or Pro tier, you are able to customize the CSS fully in your chat.
If you need any assistance with this, feel free to open a new topic, and explain what you're trying to do, and we will be happy to assist you. The benefit of a public forum setup like this, is that others can also find your topic later and find answers without having to ask themself. Win-win!
If you have any suggestions too (for example, if you want a specific CSS class added to the chat for a specific situation), you can suggest it here, too.