Well, I feel exactly the opposite. Although, I had no chance to write gtk theming engines due to lack of neccessity, I would argue that most of the time clear good ol' imperative chunk of simple and straightforward C is far better than arcane declarative CSS based on whatever underlying rules I am not fully aware of.

P.S. I has experience with both - besides writing my decent share of C code over the course of past 10-or-so years, I had an opportunity to write stylesheets in pre-HTML5 times (for money). Later was far more sane and flexible (yet a bit arcane due to multi-browser compatibility) than writing on pseudo-CSS in order to compactife modern Eclipse (4.x). No way I would believe that pseudo-CSS for constantly unstable underlying engine could be somehow better than "clunky, complex loadable modules in C".