Add an option for using a background for fringes

Thanks for the beautiful themes! Finally I can quit the “yellow background” (I have used solarized and gruvbox) and use a beautiful and usable theme with white background (and lower my screen brightness slightly).

One thing that I missed from other themes is that I like the the fringes to be visible and thought that this could be an option. I don’t know if this aligns with what you want with the theme and if bg-inactive is the most suitable color, but it looks good to me. Feel free to merge, reject or change as you see fit.