Well ok, you could A/B test replacing Arial by Helvetica, but it would be pretty meaningless and probably not make any difference. That doesn't mean that there is no difference. Maybe on a subconscious level, people would start to perceive your site and relate to your brand differently.

And maybe changing the font, and then the background separately would not make a difference, but changing both at the same time would make an impact. All those things are A/B testable in theory, but I don't think it would be very effective.