Adds an option to disable custom board stylesheet

I also expanded the options window to make room for the new check box.

I noticed that the "active_page" variable isn’t utilized by all of the pages; for example, the board browser and the rules page of a board are both listed as “page.” Is this the correct way to tell which type of page the user is on, or should I use another method? I didn't want to remove the checks to work around this because that would cause errors.