The content model for template styles sheets isn't correct. Just created [[ https://no.wikipedia.org/wiki/Mal:Example-template-styles.css | w:no:Mal:Example-template-styles.css ]], and given the `.css` suffix it should be given the //sanitized-css// content model. On inspection it has the //wikitext// content model.
TemplateStyles is just enabled (T193786) and I guess it is just some additional configuration that is needed.
I have requested a manual change of content model, so it might have changed when someone checks this report.