Theme shortcut

I would appreciate a shortcut to switch themes. I frequently use sepia and dark theme and would like to switch between them more easily. As the theme setting is not accessible via the menu, it is not possible to set up a OS X shortcut manually. It would be nice to either have ‘native’ shortcuts or, at least, to have the theme settings somewhere in the menu. Thanks!

PS: I :heart: FoldingText and use it as my one-for-all writing tool!