Fixes an issue where previously the background color would be incorrect
when the color scheme was changed, which was particularly noticeable on
Phosh where the background would display when toggling the on-screen
keyboard.
Note that the logo has been intentionally left out this time to simplify
things a bit.
This fixes an issue where previously the derivation had to be evaluated
before importing the base16 scheme, thus causing `nix flake check` to
fail when multi-platform support was added.
See: https://github.com/NixOS/nix/issues/4265
