public function test_save_changeset_post_with_theme_activation() {
    global $wp_customize;
    wp_set_current_user( self::$admin_user_id );

    $preview_theme      = $this->get_inactive_core_theme();
    $stashed_theme_mods = array(
      $preview_theme => array(
        'background_color' => array(
          'value' => '#123456',
        ),
      ),
    );
    update_option( 'customize_stashed_theme_mods', $stashed_theme_mods );
    $uuid         = wp_generate_uuid4();
    $manager      = new WP_Customize_Manager(
      array(
        'changeset_uuid' => $uuid,
        'theme'          => $preview_theme,
      )
    );
    $wp_customize = $manager;
    do_action( 'customize_register', $manager );

    $manager->set_post_value( 'blogname', 'Hello Preview Theme' );
    $post_values = $manager->unsanitized_post_values();
    $manager->save_changeset_post( array( 'status' => 'publish' ) ); // Activate.

    $this->assertSame( '#123456', $post_values['background_color'] );
    $this->assertSame( $preview_theme, get_stylesheet() );
    $this->assertSame( 'Hello Preview Theme', get_option( 'blogname' ) );
  }