Add classDark and classLight to css.ChromaStyles and "gen chromastyles" (default dark/light) #15167

I argued with @jmooring that it would be very unlikely that e.g. the CSS dark class would collide with existing classes in a project. After thinking a little, I still don't think that is a likely issue, but:

1. I suspect there are JS/CSS frameworks with other naming standards for these mode classes (e.g. theme-dark)
2. I would be an annoyed Hugo user if I needed to customise this class and had to do ugly string replacements.
3. The cost for us adding these 2 new options is small.