If they do not mind introducing C++ (they're introducing Rust so i guess multilanguage development isn't out of the question) then FLTK could be an option, though it'd probably need to improve its theming support.

They both have kinda similar roots in that XFCE originally used XForms which was an open source replacement of the SGI Forms library while FLTK also started as a somewhat compatible/inspired opensource replacement of SGI Forms in C++.