Niji is an extensible theming framework that brings uniform, responsive and comfortable theming to the tinkerer's desktop. It currently comes with builtin support for GTK apps, sway, hyprland, kitty, and others, but it also allows you to easily add custom modules for anything you desire. Happy hacking!
Arch users can install niji via the niji-git AUR package. A guide for manual
installation can be found
here.
For documentation and a guide to get started, see the docs.