This looks like a good structured way to learn Go. Is there a way to turn the lights off on these GitBooks?
The project is open-source (https://github.com/quii/learn-go-with-tests) so there's a few options available to you.
In releases, you'll find PDFs and epubs, I'm pretty sure most epub readers will let you use a dark mode. Or you can just read the markdown files on GitHub, which also supports it.
Dark Reader: https://darkreader.org/