Could you add a button for this?
Thanks.
You may try this?
(I think you have to be logged in to see the screenshot.)
(http://s16.postimg.org/99uo5axdd/th1.jpg) (http://postimg.org/image/99uo5axdd/)
more info also here http://support.locusmap.eu/hc/en-us/articles/200034192-Change-Theme-of-Vector-map