Sorry to revive an old post, but I thought someone may land here (as I did) and could be interested in a possible solution.
As you, I was wondering how to deal with the colouring inside Frontail, and I found this thread. Thanks to @5iver and @marcel_erkel, who provided the first info I needed to start, I messed around a bit with the json and css files.
In case you (or anyone else) are interested, I went a bit further and documented what i did in this post.