CSS and JS appearance customization

I found a panel in the appearance section of the magazine panel that seems to allow custom CSS and JS to be used to modify the look of a magazine. Is there anywhere I could learn more about that or is it another case of the docs not being there yet or even just the feature not quite working yet?

I see you're able to change the background of a magazine, I assume that you can do the same things you used to be able to do on old.reddit? I don't have enough CSS or JS knowledge to find what can be modified, much less how to go about modifying it but maybe someone has a little bit of insight.