Is there any way of adding some html/styling to a sublist line in edit mode? This is on a custom records child record sublist. I have these default lines that has a hierarchy to them and I'd like to show that in a nicer way. Instead of record 1 record 1 : record 2 record 1 : record 3 record 4 record 4 : record 5

Unless you are viewing this in a place that you have full control of, such as a SuiteLet, I'd say no, that's not possible. Someone else might be able to tell you a weird, "hacky" way to do it with inline JS / JQuery but I'd avoid that.

Yeah, I'm trying to avoid going the DOM-hacking route. But I might resort to that in the end.

You might find that the sublist elements lack id's, I'd make a point of inspecting the DOM before you invest any time in it if you take that route.

For now, I settled with having an edit-sublist that is rather ugly. But in view mode it is replaced with a custpage-sublist that has some html in a field of type textarea which looks pretty good.