Similarly, you're right - elements of a design do not exist in isolation. However, any design will have a series of choices that resulted in its various elements. Some choices are small/minor enough that there exist other elements that also work together with the whole, and so A/B testing them isn't some insult to the idea of design, but an at least plausible decision-making tactic.
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.
Sure, changing the font may be meaningless, but at least with a test, you KNOW it's meaningless. I try to let experience and taste guide my changes, but without checking that the change is effective you're just guessing.
Design exists to further a goal. Sometimes the goal is brand related, which I'll admit is just about impossible to measure, and sometimes it's seeing if you can reduce the frequency of support requests. One of these is obviously a better candidate for A/B testing.
[Edited to fix a spelling error]