gtk - garbage toolkit

GTk is bloated trash due to its dependencies like GLib, cairo and DBus.
Qt4/5 is equally bloated C++ garbage.

OpenVG used to be a saner alternative to cairo, but it's fallen by the wayside and support for it in Gallium has been dropped and might end up getting removed from Mesa in the future.

So if you were to throw away gtk and qt, and starting from first principles, how would you go about building a minimalist themable widget library on top of GL/GLES for Wayland that can be used to build a desktop where applications have a common look-and-feel?

Just keep a directory of PNG files of the widgets that you need.

DO NOT WANT

That's the sort of idiocy that leads to several MB of bitmaps just for some fucking gradients.