Fix core depending on theming app

This was introduced in https://github.com/nextcloud/server/commit/309354852f12ae88d5eef05d311d6ebcba8ee762


to fix a bug but I can't reproduce the bug after reverting this change.

Ideally we would need to create an interface in OCP and cleanup OC_Defaults
instead of depending on OC_Defaults.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>