Local tasks sometimes appear unstyled

Since the theme registry is cached as a whole we cannot put conditional logic in hook_theme_registry_alter():

  // The following does not work
  if (!$admin_context->isAdminRoute()) {

Currently it happens that, after a cache rebuild from the backend instead of e.g. drush, the changes to the theme registry are not applied, so leading to no theme overrides, and thus no CSS styling applied to the local tasks.

Instead, we have to move the conditional logic to the presentation layer, e.g. using a theme suggestion or the like.