Bugfix: Theme loading without matching theme

We've fixed an issue where theme loading would break when there was no matching oC theme found for the user's OS setting. For example, this occurred when a user's OS setting was configured to "dark," but the instance of oC did not offer a dark theme.

https://github.com/owncloud/web/issues/10657
https://github.com/owncloud/web/pull/10659
