Remember light/dark mode user setting

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• [ ] Create a user preference.
• [ ] Default to system if the preference isn't set.

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Maybe use the one from this Tailwind UI template.

Syntax - Tailwind CSS Documentation Template

https://syntax.tailwindui.com/

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Maybe just do the thing from the Tailwind UI theme I'm using for Livie's site and have it be a local storage setting and not a database thing at all.

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There's also the option of using a session like we do the sidebar.