I would bet that the easiest thing would be FLTK. You can use system colors and make an openGL window while having everything else you would expect in a UI lib

I would bet that the easiest thing would be FLTK. You can use system colors and make an openGL window while having everything else you would expect in a UI library. Executables with no dependencies start at a few hundred KB and it is very fast. There is even a single big pdf for documentation and dozens of examples that show off every widget. FLTK is very underrated these days because no one is out there marketing it. GLFW and IMGUI are great libraries, but once you start wanting a program with various modal and non-modal windows, menus, widget layout, a native look, file dialogs, fonts, drag and drop, copy and paste, unicode, spreadsheet tables, physical printing, custom widgets, functions to hide OS specific system stuff (like stat, directory listing etc.) you might wish you had started with something made for what you are trying to do.