(Asking as someone who wants to create website) How to make website support dark mode without javascript?

CSS prefers-color-scheme

This should help you

https://css-tricks.com/a-complete-guide-to-dark-mode-on-the-web/

If you're not wanting to use JS then you're reliant on the users browser supporting prefers-color-scheme (caniuse) or forcing them to reload the page.

Use CSS

<a href="dark-page.html">Dark Mode</a>

You can style both dark mode and light mode with CSS but the question is how will you toggle between them? A button? Then you need JavaScript

Locking as this post violates rule 5.

Just use JavaScript.