Perhaps https://phabricator.wikimedia.org/T91201#8545267 (see prototype) would help with this?

That looks really excellent.

Please experiment with how far that is away from the edge of the corner, the lower-right corner can be stolen by other things on a screen (pulling up a bottom bar on the entire OS interface for instance). I would make the text for this particular option clearer: "limit page width" / "expand page width" so that it is unambiguous.

(perhaps to someone working on page layout, it's clear that the 'page' is always full width and its the body content width that's at question, but it really isn't clear to me. and 'narrow' would be more clear to me than 'limited' but I sense there is opposition to using that word?)

Thanks for the motivating words @Sj . Mostly focusing on the technical aspects of this right now before putting it in front of a designer. I've also updated the demo to test out the possibility of dark mode in case you are interested. More information on T91201.

Marking this as a subtask of T91201: [GOAL] Accessibility settings/preferences on desktop and mobile for now

Updated description -- the page-width and persistence issues have been addressed. Still keen to see the latest v of your prefs idea jdlrobson :)

The problem here was hopefully solved by T311160.