Change the background color in slideshow mode

[Flurrywinde]

I know there's an indicator in the bar whether sxiv is in slideshow mode or not, but I'd like a more prominent indicator, such as the background color. The place to do this seems to be ci_slideshow(), and I think it's win_alloc_color() that will do it. However, I couldn't get it to work. Can win_alloc_color() be called in ci_slideshow(), and if so, how? I couldn't figure out how to pass &win->env and &win->bg to it in the right way. Or is there a better or more proper way to do this?