I already searched for ways of how to do it and I keep on seeing others deployed their own frontend on Vercel / Netlify, just to add a toggle switch for dark and light mode.
I think this is a very basic feature and there should be a way to show the toggle on the default Gitbook theme. But after digging for more than 24 hours I can only set it to light / dark mode in app.gitbook.com. There's no way for visitors to toggle between light and dark. Or maybe there's a way and I just can't find it.
GitBook has recently just launched this feature natively!
https://changelog.gitbook.com/may-2023/30-may-a-light-dark-mode-toggle-and-new-customization-options