Support for only-light and only-dark classes

[choldgraf]

Context

Recently we had a conversation where @larsoner shared some JavaScript to swap out images depending on the theme that was active. Another pattern that is out there is using CSS classes, as Furo does. Basically you define a class that makes things only display when one or the other theme is activated.

Proposal

We should add CSS rules for the only-dark and only-light classes, and document their use here. We should try and follow whatever standard Furo set since it seems reasonable and so that we don't create too many different ways of doing the same thing.

Other things we could do with this:

• Add theme support for html_logo_dark

[larsoner]

Indeed I'd rather use this than JS!