I use version 1.7.5 clean install.
But, when I add the “Learn2” theme, the theme is not visible in admin > themes.
I have added it via the admin panel first. Afterward I have added it via cli (bin/gpm install learn2), but it is still not visible.
There are some extraneous tab characters in the theme’s configuration file located at user/themes/learn2/learn2.yaml. They prevent the theme from loading and showing up in the admin interface. The culprit is the root_page entry. Remove the tab characters there and you should be fine.