[Theme: The Erudite] How to offer visitors the option to change the css to dark mode.

You need theme switcher code or a plugin. A google will provide, but you will probably find this a good place to start: wordpress article on theme switching.

However, I suspect the dark/light schemes are exactly the same theme with a simple CSS tweak. Look at the css file in the admin panel to see, you can include some very simple php code to set the appropriate switch I am sure.