Dark mode is easily toggled within Admin 2020 and is saved in the user preferences.
To enable dark mode, click the user icon in the top right of the admin bar
You should now see the flyout user menu. Here you can see an overview of your site as well as any notifications you may have. Click on preferences and you will now see the dark mode toggle.
Once enabled, you site will switch to dark mode instantly and your preferences will be saved.