Note that if you have the site setting default dark mode color scheme id configured, dark/light mode should be determined by the user’s browser or operating system preference… so in that case you might not need to do anything additional.
Beyond that, there’s not a great way to do this at the moment. A user doesn’t have an account at this point, so there’s not an easy way to toggle between light/dark (as a user preference).
One minor improvement that I’ve just put in our review queue is to add a class to the body tag (.invite-page)… once merged this will make it much easier to add some custom CSS to target only this page.