Comment 5 for bug 1960358

In this specific customer usecase, they would like to have the custom theme as the default and don't want to let users change the theme to other ones.

I agree that we shouldn't delete anything from the file system but to use AVAILABLE_THEMES to tweak the list of the themes to be displayed. In fact, the theme here has "@import "/themes/default/variables";" statement so if the default theme is deleted from the filesystem, Horizon gives 503.