Here’s what I use, with localStorage instead of cookies to remember user setting: https://hypertexthero.com/dark-mode-website-theme-switcher-l...
I'm not aware of the specifics of regulations in other parts of the world, but this distinction is irrelevant for the EU ePrivacy directive: a) it was never about cookies only, and b) purely functional cookies that record user preferences do not need explicit consent.
From [0]:
> 34,35 Storage of information in the sense of Article 5(3) ePD refers to placing information on a physical electronic storage medium that is part of a user or subscriber’s terminal equipment [..] this includes making use of established protocols such as browser cookie storage as well as customized software, regardless of who created or installed the protocols or software on the terminal equipment.
> 43 This might also be the case for web browsers that process information stored or generated information [sic] inside the device (such as cookies, local storage, WebSQL, or even information provided by the users themselves). The use of such information by an application would not be subject to Article 5(3) ePD as long as the information does not leave the device
https://www.edpb.europa.eu/our-work-tools/documents/public-c...
Doesn’t bother me enough to change the technique at this time, but I’ve put it in the things to do list.
I like the idea of having it automated by the system-set user preference in addition to having an option to set it manually with a link.