Assert a theme is installed, or is the active theme, as the Themes screen lists it. The theme is named as the screen shows it.
Examples
Then the "Twenty Twenty-Five" theme should be active
And the "Twenty Twenty-Four" theme should be installed
Then the "Twenty Twenty-Four" theme should not be active
And the "Some Theme" theme should not be installed
Then the "Twenty Twenty-Five" theme should be installed
Pattern
Then(/^the "([^"]*)" theme should( not)* be (installed|active)$/, …)
Part of @webship-js/wordpress-steps (github.com/webship/wordpress-steps), a step pack for Webship-js 2.0.x.