Per-page light / dark fixups

[lmktfy]

Let's say you have a website that was made with a dark background assumed.

For the docs section, you (as site owner) want to allow toggling between light / dark / auto. But you also have a small number of pages where you want a compromise theme used, because they don't display well in light mode or auto-light mode.

Eventually you hope to fix up those pages, but for now you need a workaround. Either a front matter marker or a shortcode that wraps the problem content, such as an image.

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This feature request is to add that support, both ways round (pages that assume light themes, and pages that assume a dark theme).