Previously the space was sanitized away by `fetchurl` because it was
part of the derivation name. Also, the path with a space was not used
in the `schemeless` theme, which is where the bug happened.
- Converted stylix config into full modules that define `stylix`.
- Introduce `images.nix` for defining common images.
- Renamed `testcase` → `themeName` and `stylix` → `themeModule`.
Co-authored-by: Flameopathic <64027365+Flameopathic@users.noreply.github.com>