To turn it on go to user settings by clicking your name, hit the dropdown for Preferred Theme, and select "comment-boxes".
Unfortunately user themes are overridden by the subforum themes, which are used to change the colors in places like /f/just_post and /f/GoldenAgeOfTV. You can turn off subform themes entirely by unchecking "Show custom stylesheets" in user settings, but that also means every forum is now blue.
I designed the comments to be dense because that's the way I like em, but we'll probably end up changing that since I understand why people would have problems with em. There's a couple ways we could build options to accommodate everyone, like let people change how comments look but still give em custom themes, but I can't promise that we'll get around to it.
Oh! And sometime soon I'll make a guide to how custom themes work, to help you make your own!
Thank you for posting! Let me know what you think.