Thanks for the feedback!

... why not just login on mobile? Wouldn't that solve it for you, personally?

Yes, it would; that I'm not logged in by default on mobile is also a matter of personal preference: since a mobile device is something I can theoretically lose more easily, and despite the fact that my mobile is of course password protected too, I still prefer to reduce the number of things that are permanently logged in on it. (I also very rarely post from mobile because I find it more tedious to type on.)

I would say my proposal has the advantage of also respecting the preferences of all other anonymous visitors with this option turned on - how many there are I don't know of course, but I'm betting it's not zero. And even though it's only a small thing, considering how relatively easy it should be to implement (I hope), I think it's a nice thing to do for new visitors.

... there is no way I can miss being logged out. I appreciate that this is almost certainly not a common enough accidental use case of how things are to count against the proposal.

I'd be happy to support a variation of this where there are two different dark themes with different foreground colors in order to make it easier to tell the difference between being logged in and logged out (though personally, my nodelet configuration is different enough from the default that this would probably be enough for me to differentiate).

Another advantage of my suggestion is that it can help even logged in monks: Say for example I want to use the system-wide light theme during the day and the system-wide dark theme in the evenings, I can do so with a single preference change in my OS, and the setting immediately propagates everywhere. Without an "auto light/dark" theme on PerlMonks, I would have to go into my display settings on PM every time, which is (relatively) cumbersome. (This of course doesn't require a change in AM's settings, that could come later after sufficient testing of the new theme.)