Add classes for multi-theme content support

[12rambau]

FIx #658

As suggested by @choldgraf, The theme is now allowing the devs to use only-dark and only-light to use different images based on the selected mode.

It works with any elements (I'll use it for example to render multiple instance of jupyterlite in one of my documentation). Thus it's not documentated in images section but in the customization section.

Let me know if you think about any loopholes.

PS: the doc demo: https://pydata-sphinx-theme--673.org.readthedocs.build/en/673/user_guide/customizing.html#use-specific-content-in-each-theme

[choldgraf]

This is looking great to me - one quick idea, though I think it'd be ok to move this to another PR: I think the most common image people will want to use for this is the logo in the top left. So I wonder if we could support another top-level config like "html_logo_dark", and place this next to the current logo, and add the respective theme selectors. That would allow users to have light and dark logos

[12rambau]

I was thinking about that as well but I think it deserves it's own PR if you could open another issue and assign it to me so that I don't forget that would be perfect

[choldgraf]

Done here: #674 !

[choldgraf]

I cleaned up the explanation a little bit - I hope that's OK!

It looks good to me though 👍

[choldgraf]

Hmm can we choose a different sample image? The daily-cat image doesn't seem to resolve anymore: https://source.unsplash.com/200x200/daily?cute+cat

[jarrodmillman]

Hmm can we choose a different sample image? The daily-cat image doesn't seem to resolve anymore: https://source.unsplash.com/200x200/daily?cute+cat

It appears to work for me.

[choldgraf]

huh weird - maybe my internet was just messing up...OK then I think we are good to go here :-)