This is a theme for gtk3 with a focus on code simplicty.
Did you ever want to fix one little issue in a gtk3 theme and instantly gave up once you saw the code? That is at least my experience. So instead of tweaking an existing theme to my liking I decided to write one from scratch.
There is not a lot of documentation on writing gtk3 themes. It is especially hard to know which selectors can be used to style an element. So there may well be some selectors I missed.
I tried to make the code as clear as possible. So maybe it can serve as a reference for people who want to build their own themes.
And IMHO it also looks ok, btw.
Update
In the meantime, the situation has improved a lot. The documentation is
much better, the reference implementation is much cleaner, and there even
is an interactive inspector ($ GTK_DEBUG=interactive {your-app}).
On the other hand, the concept of GTK themes itself is highly contested.
Anyway, I think there is still value in a minimal theme that is easy to modify. So I will keep using this.