Assert which theme is the site's default theme, by the name the Appearance page (/admin/appearance) shows next to "(default theme)".
Examples
Then the default theme should be "Olivero"
And the default theme should be "Mercury"
Then the default theme should not be "Bartik"
And the default theme should be "Claro"
Then the default theme should not be "Stark"
Pattern
Then(/^the default theme should( not)* be "([^"]*)"$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.