The base theme stable9 is not enabled when updating the theme on Drupal 9.x

same goes for current stable - it does not work - and for the same reason

Can you please provide reproduction steps. I was able to enable this on Drupal 10 fresh install.

I see the same issue. There appears to be a dependency on the stable9 base theme, that is a contributed theme. But, it is not defined in the composer file for this theme. So, when you install the theme using composer, you will start to see the Base theme stable9 has not been installed. (Drupal\Core\Theme\MissingThemeDependencyException) exception in watchdog.

It looks like the stable9 base theme dependency was added in issue #3162911 ✨ Use stable9 as base theme Fixed

I was able to resolve this issue on our site by enabling the stable9 theme (which is a hidden theme in the latest version of core) via drush drush theme:enable stable9. I wonder if there is a way to add the stable9 theme as a dependency that will force the stable9 theme to enable when a user upgrades to the latest stable version of the Bulma theme?

Themes cannot have their own update hooks so not sure how to fix it. Hmm maybe there could be a contrib module that every theme depends on? Only just thought of that— really not clear on how this major issue isn't a solved problem Drupal-wide!

Anyhow here is is what we have in Drutopia profile's install file:

/**
 * Uninstall stable (8) theme and install stable9, what Bulma now requires.
 */
function drutopia_update_9101() {
  /** @var \Drupal\Core\Extension\ThemeInstallerInterface $theme_installer */
  $theme_installer = \Drupal::service('theme_installer');
  $theme_handler = \Drupal::service('theme_handler');

  $def_theme = $theme_handler->getDefault();

  if (in_array($def_theme, ['octavia', 'bulma', 'camouflage'])) {
    if ($theme_handler->themeExists('classy')) {
      $theme_installer->uninstall(['classy']);
    }
    if ($theme_handler->themeExists('stable')) {
      $theme_installer->uninstall(['stable']);
      $theme_installer->install(['stable9']);
    }
  }
}

#8 worked for me. Thx!