> This is more of an indictment of the way GTK3+ handles theming (or lack of it).

How do you think GTK+3 handles theming (or lacks handling for it)? It sounds like you're confusing GTK+3 theming for GTK+2 theming (which was based on "engines", which were quite a mess). Now it's very "proper and defined", it's accomplished with a strict subset of CSS which has fairly consistent behaviour.

I personally don't see the point in elaborate theming systems (I'd prefer to just have control over the colours, and maybe one or two of the spacing variables), and it is obvious that they cost a lot of cycles at run time....

but GTK+3's theming system is not braindead, it's just ambitious.