I’m tempted to go Nuklear
Nuklear is a library that is a single-header ANSI C immediate mode cross-platform GUI toolkit. It lets you develop stylish and beautiful GUIs for your applications. It’s written in C but any programming language that can work with C such as C++ or Python can also use it.
Importantly, it also includes documentation so you can make use of it. The best software in the world is useless if you can’t use it and GUI toolkits tend to be a little bit more complicated than say a text editor. This is nicely written, and though it’s just one document, it’s a long one!
It’s one thing to write a simple GUI as I did in that Empire game but mine was only 600 lines long and pretty rough looking. Nuklear is 18,000 lines long ie 30x as big. If there’s one thing I’ve found from my software development experience, it’s that a nice looking piece of software will get away with having more bugs than something that looks not as nice.