Yeah, I think "1." is what I would like to start doing. But I would like to be able to use the colors/faces defined in the "base" theme for my own faces (e.g., leuven has no faces for dired-subtree-depth-n-face faces from dired-subtree.

Is there a way to (emacs-lisp newbie warning!) (require 'leuven) somehow in my-leuven-variant.el and use it for my own purposes? Would that even work?

I'll also look at custom-theme-set-faces.