ethicalads: follow Material/Zensical dark mode - #716

Root cause: we add the adaptive-css class to the placement, but the EthicalAds client's adaptive mode only reacts to markers it knows on html/body — the dark/auto classes, data-theme, or data-bs-theme. Material-family themes (Material for MkDocs, Zensical, sphinx-immaterial) signal dark mode with data-md-color-scheme="slate" instead, so no selector ever matches and the ad keeps the light default.

It seem this should be implemented in ethical ad client instead so the adaptive-css works out of the box, right?

One possibility since there is an unlimited number of attributes where the light/dark mode could be set is to set the attribute on the ad placement (eg. data-ea-adaptive-attribute). Then the ad client could monitor that attribute for changes and trigger some sort of update code.

Could be in either, we should just fix it somewhere because it currently looks pretty bad :(

I really wish there were standards here. :(

Unfortunately, there isn't a standard here.

Looking more closely at what Zensical is doing, they're setting data-md-color-scheme="slate" or data-md-color-scheme="default". Our normal light/dark selectors wouldn't even work. There's a few possibilities:

1. We could just handle Zensical's settings. We would just add body[data-md-color-scheme="slate"] here. This is straight-forward but a one-off fix for them.
2. We add an attribute like data-ea-dark-selector="body[data-md-color-scheme='slate']". The client sets up a MutationObserver to watch for changes to that selector. When it changes, we'd have logic to evaluate whether the ad should be dark mode or not and then apply the class manually. It's more work but it would allow this to work on any theme as long as that attribute is set. I added a not fully tested draft implementation here.