made themes selectable again

I added the flask url_for wrapper for the diferent themes css rel's to they can be switched again.

I also fixed an issue with the slate theme (because thats the one I use), though it exists on a few of them. On the adventure map, the text fades to white. On the themes that don't have a white background for .modal-content it doesn't have the intended effect. Sort of tedious to fix but at least include the one for slate.