Right click on the thing you want to change and choose Inspect. That will show you the current class/id to key in on. I haven't seen docs on that. it could also be a moving target since kbin is still pretty new and under very active development.

I haven't seen any documentation but I'm sure it's just the last CSS loaded. So if you know how CSS works, you could inspect the page and write CSS attritubes for the tags/classes you want to modify.