That would be nice. I don’t know if that involves much change in to code. Just to rephrase you point little bit: the idea of having a button in card template editor to toogle the preview windows to a nightmode preview, so not changing the whole Anki IU.
That woud prevent restariing Anki each time for the sole purpose of changing the nightmode on and off in order too see the styling. That scenario would be more useful for the users who do review almost exclusively on mobile versions with nightmode on rather than in the desktop. Just my thought.