TemplateStyles was deployed to the German Wikipedia in T190910. The deployment went fine, but the creation of TemplateStyles CSS pages on the German Wikipedia is a bit awkward.
Subpages are, in a technical sense, disabled in the template namespace on the German Wikipedia; $wgNamespacesWithSubpages[NS_TEMPLATE] is set to false. It was in the first commit in the configuration, and there's no comments, so there's no clear reason why this is the case. A page like Template:Foo/styles.css is not technically a subpage of Template:Foo, but a totally separate page. This doesn't cause any real problems normally, but in the case of a page like Template:Foo/styles.css it means the content model has to be manually changed by a user who can do that, rather than the content model automatically being the correct one. It's a bit of a pain.
The most sensible way to solve this problem is probably to remove the override of $wgNamespacesWithSubpages[NS_TEMPLATE], since users of the German Wikipedia already do use subpages in the template namespace, which will mean TemplateStyles will then work more sensibly for people.