[aaronmdjones]

Actually I'm betting on the more knowledgeable to point out that browsers signal light/dark theme preferences automatically, without cookies.

https://drafts.csswg.org/mediaqueries-5/#prefers-color-schem...

https://caniuse.com/?search=prefers-color-scheme

[jessaustin]

Yes, that exists, obviously. However, it's perfectly valid for someone to prefer a particular site to override an OS or browser setting.