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

@chakkche it looks like there are still a number of failing tests, such as:

testUninstallingThemes()
InvalidArgumentException: Input "admin_theme" cannot take "stark" as a value (possible values: "", "claro", "olivero").

AddedStylesheets
The link Set Stark as default theme was not found on the page.

StandardPerformanceTest
Failed asserting that two arrays are identical.

Drupal\Tests\navigation\FunctionalJavascript\Performance
Failed asserting that two arrays are identical.

I wonder should we leave Stark as an available theme on the Appearance page, or should we update these tests? Given the point of Stark, it looks like the tests should remain as is and use it, but I'm not sure how to get around that being the case _and also_ hiding it from the Appearance page.

@markconroy i suggest we create a test_functional_appearance theme in system and extend stark as base theme and use it as replacement in all the tests wherever stark is still needed.