Support the notion of light and dark themes

[choldgraf]

A lot of modern websites have some notion of a light and a dark theme, and many Sphinx sites are starting to implement this as well. However, building the proper HTML structure light and dark is kind of tricky and unintuitive for people, though some patterns to do so are fairly standardized. This makes me wonder if the basic-ng theme could be one way to provide components, classes, etc that sub-themes could use to follow best practices in light/dark themes.

A few examples:

• In the pydata theme we had requests that the logo needed to have a "light" and a "dark" version for packages that didn't have a "dark-friendly" logo by default (as an aside we also needed to have both a logo and a title which may be pretty common for others as well)
• Using standard class names like light-only and dark-only

...I'm not sure what else, but wanted to throw this idea out there in case it resonated with anybody else. I'm not sure how to encourage best practices without getting too opinionated about what the end-result will be 🤔

[pradyunsg]

I think it makes sense to provide resources / examples for how to do this, so that theme authors can implement this. I'm torn on trying to add dedicated support for this, since:

• each theme could want to do things slightly differently.
• this needs JS support logic which I really don't want to bake into this theme. (https://sphinx-basic-ng.readthedocs.io/en/latest/goals/#what-it-does-not-provide)

[pradyunsg]

And I'm also fine with saying "hey, do it this way so that it's portable across themes"?