Add default theme config option

[davidovski]

Just realised that I had already implemented #135 on a private branch and forgot to mention it or make a PR for it.

The way I did it was quite hacky just to make it work for my instance but I've just updated it so it should work for any instances

Pretty sure it works well, I tested it with different default themes, and none being set and it seemed to work as expected.

Just realised that this is a duplicate PR to #139, so if you want we can compare how we've implemented it and get best of both here.