I'd like to change the background color of the fullscreen mode.
Make fullscreen background colour configurable.
I've been patching sxiv to use WIN_BG_COLOR instead of black for a little while now, but I'll offer a patch for configurable background colours so I won't need to update it when I upgrade ;)
FWIW, if you just want to use your configured background colour in fullscreen mode you can simply remove the conditional in window.c:win_init:
XSetForeground(e->dpy, gc, win->bgcol);
Fixed issue #41, added WIN_FS_COLOR to config.h