Consider adding option to disable night_mode class

[krassowski]

Excerpts from #37 :

r: Basically, what I'd like is a way to use your plugin as a night theme for Anki that doesn't actually affect the way cards are rendered in any way – that is, a way to selectively disable night_mode only for the cards. (I guess you could however indeed make that the responsibility of the user, leaving it to them to remove any night_mode CSS instead if they so want).

k: I can imagine that someone got an Anki deck with pre-defined CSS styles which do not match their needs on PC, however, these work well for them on mobile. If the hypothetical user does not know CSS, it might be a good idea to let them disable this feature (as the styles defined for mobile might conflict with the PC needs and such users would not create their own styles anyway).