In my D8 site I have Magazine Lite, a sub theme of Classy. Magazine Lite is installed.
In web/themes/custom I have a theme installed that in its .info.yml file names Magazine Lite as its base theme.
On the Appearance tab, instead of an install link is the message:
This theme requires the base theme Magazine Lite to operate correctly
In what way does it require the base theme beyond the base theme being present and installed?