Hide Stark from the Appearance page UI

@markconroy in some of the tests default install theme is Stark. Should we change it to Claro or Olivero as default install theme.

Stark is perhaps useful for the tests as the markup is very simple, maybe we just need to install the theme in a different way in these tests (unless the tests are testing the theme installer itself)

@longwave We're observing that the Stark theme is being installed in the backend; however, we are hiding it from the Appearance page in the Admin UI. As a result, browser tests such as BlockTest, BlockHiddenRegionTest, ThemeTest, and a few others are failing because they rely on Stark theme-related text that is no longer visible in the UI.

@chakkche Let's keep Stark as the theme we want to use for the tests. But update the tests.

Also, let's have a read of the linked issue that this issue was spun off from to see some of the approaches that have been made towards this already. 📌 Discourage use of Stark as a base theme (and possibly mark it internal or hidden) Needs work