If your intention is to let the user choose between styles the best way would be Alternate Stylesheets: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/... . Sadly Google can't be bothered to implement "legacy" standards (=standards not invented at Google).
If your intention is to adopt your styles slightly to user preferences then there are media queries.
Alternate stylesheets don't persist (at least in Firefox), which is really annoying.
Media queries are really nice though. Being able to provide a night mode alternative without even asking the user, assuming they've set the OS to their preference, is really seamless.
> Alternate stylesheets don't persist (at least in Firefox), which is really annoying.
Yes, I also think they should. What I wondered, is this behaviour prescribed somewhere? Even if it were, form content is retained even across randomly typing in a different URL and navigating back, form submission and even across real crashes just seconds after I typed the letter. This is really nice and I don't think it is mandated. This is the stuff User Agents should be compete with, not ways to get data from the User to a surveillance company.
> Being able to provide a night mode alternative without even asking the user, assuming they've set the OS to their preference, is really seamless.
Yes, that's nice and how every thing in a UI should be. It helps both the programmer and the user in efficient and controllable usage.