The limechat theme I use.

README.md

Link or copy the css and yaml files into "~/Library/Application Support/LimeChat/Themes/"

Then choose "isaacs" as the theme in preferences.

It's a lot better if you install the included CentSchBook Mono font, which is also just the best monospace font ever. But really, any font will probably do, I'm just most used to looking at this one.