Using bgClass instead of hardcoding to .reveal-modal-bg

This caused problems for me because I'm using my own background class, with my own div on the page. When `toggle_bg` is called, it looks for `.reveal-modal-bg` and of course doesn't find it because that's not the class in use. I don't see any reason to hardcode it to look for `.reveal-modal-bg` instead of using `bgClass`.