Pretty sure the point of using the rendering engine to display the UI was to make it easier to write cross-platform UI; granted, though, the point of then having the feature to use CSS to modify the UI was to do that (and it was relatively easy once the UI was already written in the rendering engine).

I actually had changed some about:newtab styles. They even outlived one or two updates after that, until full reset.

Except I don't think it is even documented anymore?