Subthemes use graupl settings

Problem/Motivation

The sub-theme settings are mapped to the graupl settings.
You can not change the subtheme settings to a value different than the graupl settings.

Steps to reproduce

1. Change a setting in the graupl base theme under /admin/appearance/settings/graupl..
2. Now go to your subtheme settings.
3. Notice that the setting set in graupl is now set in the subtheme.
4. Try to set a value in the sub-theme settings and click save.
5. Notice that the setting value is reset.

Remaining tasks

- [ ] Make the subtheme have its settings.