assertThemeExists example

    $starterkit_info_yml = $this->getWorkspaceDirectory() . '/core/themes/starterkit_theme/starterkit_theme.info.yml';
    $info = Yaml::decode(file_get_contents($starterkit_info_yml));
    $info['version'] = '9.4.0';
    file_put_contents($starterkit_info_yml, Yaml::encode($info));

    $process = $this->generateThemeFromStarterkit();
    $result = $process->run();
    $this->assertEquals('Theme generated successfully to themes/test_custom_theme', trim($process->getOutput()), $process->getErrorOutput());
    $this->assertSame(0, $result);

    $theme_path_relative = 'themes/test_custom_theme';
    $info = $this->assertThemeExists($theme_path_relative);
    self::assertArrayNotHasKey('hidden', $info);
    self::assertArrayHasKey('generator', $info);
    self::assertEquals('starterkit_theme:9.4.0', $info['generator']);

    // Confirm readme is rewritten.     $readme_file = $this->getWorkspaceDirectory() . "/$theme_path_relative/README.md";
    $this->assertSame('test_custom_theme theme, generated from starterkit_theme. Additional information on generating themes can be found in the [Starterkit documentation](https://www.drupal.org/docs/core-modules-and-themes/core-themes/starterkit-theme).', file_get_contents($readme_file));

    // Ensure that the generated theme can be installed.     $this->installQuickStart('minimal');
    $this->formLogin($this->adminUsername, $this->adminPassword);