Add a step for scenarios that need a specific set of themes

features/behat-steps.feature

@@ -542,6 +542,38 @@ Feature: Test that WP-CLI Behat steps work as expected
     And the wp-content/themes/oceanly-green/style.css file should exist
     And the wp-content/themes/oceanly/style.css file should exist
 
+  @require-wp
+  Scenario: Keep only the listed themes
+    Given a WP installation
+    And only these installed themes:
+      """
+      twentysixteen
+      twentyseventeen
+      """
+
+    When I run `wp theme list --format=count`
+    Then STDOUT should be:
+      """
+      2
+      """
+    And the wp-content/themes/twentysixteen/style.css file should exist
+    And the wp-content/themes/twentyseventeen/style.css file should exist
+
+  @require-wp
+  Scenario: Keep only the listed theme and activate it
+    Given a WP installation
+    And only these installed and active themes:
+      """
+      twentysixteen
+      """
+
+    When I run `wp theme list --fields=name,status --format=csv`
+    Then STDOUT should be:
+      """
+      name,status
+      twentysixteen,active
+      """
+
   @require-wp
   Scenario: Test custom wp-content directory
     Given a WP installation

src/Context/FeatureContext.php

@@ -1934,6 +1934,23 @@ public function install_extensions( $type, array $lines, $activate ): void {
 		}
 	}
 
+	/**
+	 * Remove every installed theme, like `wp theme delete --all --force` does.
+	 */
+	public function remove_all_themes(): void {
+		$themes_dir = $this->get_extension_dir( 'theme' );
+		if ( null === $themes_dir ) {
+			throw new RuntimeException( 'Could not find the themes directory.' );
+		}
+
+		$entries = scandir( $themes_dir );
+		foreach ( false === $entries ? [] : $entries as $entry ) {
+			if ( '.' !== $entry[0] && is_dir( "$themes_dir/$entry" ) ) {
+				self::remove_dir( "$themes_dir/$entry" );
+			}
+		}
+	}
+
 	/**
 	 * Get the directory plugins or themes are installed into, or null if it is not known.
 	 *

src/Context/GivenStepDefinitions.php

@@ -404,6 +404,37 @@ public function given_a_wp_multisite_installation( $type = 'subdirectory' ): voi
 		)->run_check();
 	}
 
+	/**
+	 * Removes all themes and installs, and optionally activates, the listed ones instead.
+	 *
+	 * Takes the same lines as `Given these installed themes:`. This replaces running
+	 * `wp theme delete --all --force` followed by one or more `wp theme install` commands.
+	 *
+	 * ```
+	 * Scenario: My example scenario
+	 *   Given a WP installation
+	 *   And only these installed themes:
+	 *     """
+	 *     twentysixteen
+	 *     twentyseventeen
+	 *     """
+	 * ```
+	 *
+	 * @access public
+	 *
+	 * @Given /^only these installed (themes|and active themes):$/
+	 *
+	 * @param string       $kind Either 'themes' or 'and active themes'.
+	 * @param PyStringNode $stream
+	 */
+	public function given_only_these_installed_themes( $kind, PyStringNode $stream ): void {
+		$lines    = array_filter( array_map( 'trim', explode( "\n", $this->replace_variables( (string) $stream ) ) ) );
+		$activate = 'and active themes' === $kind;
+
+		$this->remove_all_themes();
+		$this->install_extensions( 'theme', $lines, $activate );
+	}
+
 	/**
 	 * Installs and optionally activates one or more plugins or themes.
 	 *