This is mostly so that applications like ReGreet, which are managed by
NixOS instead of HM, can also use the same GTK settings. It reduces
repeating code by a lot and allows changing themes way more easily now.
Refactored the entire repository structure, as putting everything into
modules was just awkward. Took heavy inspiration from fufexan's
repository, fufexan/dotfiles, as the structure felt much saner.
