useDarkMode currently not working on Linux either (Ubuntu), resets on every page load (no persistence of mode preference). Using Next.js as well. For me it doesn't work in the production either.

Same thing here. Did a few tests, my guess is that the function in useIsomorphicLayoutEffect (that runs on refresh) is checking the wrong stuff (L72) before setting the local storage variable to isDarkOS (which in this case will always be true).

I think this there's a specific case that is not treated: when no value is set on local storage, useLocalStorage defaults to a true/false value based on the OS settings, instead of undefined. That ripples to useIsomorphicLayoutEffect and incorrectly checks the values and then sets the OS value.

I'm not sure what's the best way to fix this problem, since it seems to be related to other hooks and the way they handle the default values.

Hi all, thanks for the bug report, feel free to open a PR, I will check it in priority

Just created #557

The change in useLocalStorage may possibly break other hooks, but I tried to make it consistent. It still does not return undefined when the key does not exist, but now it creates the key/value in local storage when the hook is called. That asserts the local storage as the source of truth.

And that should fix the useDarkMode inconsistency too, so I removed the useIsomorphicLayoutEffect since it's now unnecessary - the isDarkMode variable is always reflecting the correct data in local storage and it does not update on refresh.