Adding your own custom logo in admin 2020 is very straightforward and can be done either by URL or uploading a logo directly into your wordpress media library.
Navigate to admin 2020 settings page and select the admin bar tab. Here you will the option to set a logo for light and dark mode.
You can either paste a URL to your logo into the field or select one from your media library.
If you don’t set a logo for dark mode, Admin 2020 will use the light logo for both light and dark modes.