This will clean up the module slightly and bring it more in line with Pantheon & Cinnamon.
While at it do some other refactoring inspired by those modules:
- Correct a typo in light background attribute name.
- Rename the attribute name.
- Quote arguments.
- Extract the overridden package list and override text into variables.
- Avoid having separate copy commands for overrides from packages.
- Avoid `with` statements.
- Use `concatMapStringsSep`.
