https://api.drupal.org/api/drupal/core%21modules%21update%21tests%21themes%21update_test_basetheme%21update_test_basetheme.info.yml/8.9.x
update_test_basetheme.info.yml | Drupal API
updatetestbasethemeinfoyml
https://core.telegram.org/type/BaseTheme
BaseTheme
Basic theme settings
basetheme
https://www.drupal.org/node/252430
Allow BASETHEME_ prefix in preprocessor function names [#252430] | Drupal.org
Jul 14, 2012 - I just noticed this from this queue on cleaning up Garland. What is the expected behavior when prefixing a base themes preprocessor with the name of the theme...
allowbasethemeprefixpreprocessorfunction
https://you-need-it.com/
BaseTheme -
basetheme