MaintenanceThemeTest.php

<?php

namespace Drupal\KernelTests\Core\Theme;

use Drupal\KernelTests\KernelTestBase;

/**
 * Tests themes and base themes are correctly loaded.
 *
 * @group Installer
 */
class MaintenanceThemeTest extends KernelTestBase {
    
    /**
     * Tests that the maintenance theme initializes the theme and its base themes.
     */
    public function testMaintenanceTheme() {
        $this->setSetting('maintenance_theme', 'test_subtheme');
        // Get the maintenance theme loaded.
        drupal_maintenance_theme();
        // Do we have an active theme?
        $this->assertTrue(\Drupal::theme()->hasActiveTheme());
        $active_theme = \Drupal::theme()->getActiveTheme();
        $this->assertEquals('test_subtheme', $active_theme->getName());
        $base_themes = $active_theme->getBaseThemeExtensions();
        $base_theme_names = array_keys($base_themes);
        $this->assertSame([
            'test_basetheme',
        ], $base_theme_names);
    }

}