Two minutes

1. Activate the plugin. That is enough: a switch appears in the bottom-right corner of every page, and anybody whose phone is set to dark gets a dark page straight away. On a first install a notice in wp-admin says so, with Preview it and Switch it off until I'm ready, which turns off Enable dark mode on the General screen until you turn it back on there. Dismissing the notice leaves dark mode on.
2. Open your site and press the switch. Look at your header — a logo is usually dark type on transparency and is the one thing that tends to disappear.
3. If it did, or anything else looks wrong, choose Fix dark mode here in the black toolbar at the top of the page and click the part. For a logo, choose Use a dark version and pick a light version from your Media Library. No selectors to type.

Everything else is optional.