* @since 3.4.0
	 *
	 * @param int $blog_id Optional. ID of the site. Defaults to the current site.
	 * @return string[] Array of stylesheet names.
	 */
	public static function get_allowed( $blog_id = null ) {
		/**
		 * Filters the array of themes allowed on the network.
		 *
		 * Site is provided as context so that a list of network allowed themes can
		 * be filtered further.
		 *
		 * @since 4.5.0
		 *
		 * @param string[] $allowed_themes An array of theme stylesheet names.
		 * @param int      $blog_id        ID of the site.
		 */
		$network = (array) apply_filters( 'network_allowed_themes', self::get_allowed_on_network(), $blog_id );
		return $network + self::get_allowed_on_site( $blog_id );
	}

	/**
	 * Returns array of stylesheet names of themes allowed on the network.
	 *
	 * @since 3.4.0
	 *
	 * @return string[] Array of stylesheet names.
	 */
	public static function get_allowed_on_network() {
		static $allowed_themes;
		if ( ! isset( $allowed_themes ) ) {
			$allowed_themes = (array) get_site_option( 'allowedthemes' );
		}

		/**
		 * Filters the array of themes allowed on the network.
		 *
		 * @since MU (3.0.0)
		 *
		 * @param string[] $allowed_themes An array of theme stylesheet names.
		 */
		$allowed_themes = apply_filters( 'allowed_themes', $allowed_themes );

		return $allowed_themes;
	}

	/**
	 * Returns array of stylesheet names of themes allowed on the site.
	 *
	 * @since 3.4.0
	 *
	 * @param int $blog_id Optional. ID of the site. Defaults to the current site.
	 * @return string[] Array of stylesheet names.
	 */
	public static function get_allowed_on_site( $blog_id = null ) {
		static $allowed_themes = array();

		if ( ! $blog_id || ! is_multisite() ) {
			$blog_id = get_current_blog_id();
		}

		if ( isset( $allowed_themes[ $blog_id ] ) ) {
			/**
			 * Filters the array of themes allowed on the site.
			 *
			 * @since 4.5.0
			 *
			 * @param string[] $allowed_themes An array of theme stylesheet names.
			 * @param int      $blog_id        ID of the site. Defaults to current site.
			 */
			return (array) apply_filters( 'site_allowed_themes', $allowed_themes[ $blog_id ], $blog_id );
		}

		$current = get_current_blog_id() == $blog_id;

		if ( $current ) {
			$allowed_themes[ $blog_id ] = get_option( 'allowedthemes' );
		} else {
			switch_to_blog( $blog_id );
			$allowed_themes[ $blog_id ] = get_option( 'allowedthemes' );
			restore_current_blog();
		}

		// This is all super old MU back compat joy.
		// 'allowedthemes' keys things by stylesheet. 'allowed_themes' keyed things by name.
		if ( false === $allowed_themes[ $blog_id ] ) {
			if ( $current ) {
				$allowed_themes[ $blog_id ] = get_option( 'allowed_themes' );
			} else {
				switch_to_blog( $blog_id );
				$allowed_themes[ $blog_id ] = get_option( 'allowed_themes' );
				restore_current_blog();
			}

			if ( ! is_array( $allowed_themes[ $blog_id ] ) || empty( $allowed_themes[ $blog_id ] ) ) {
				$allowed_themes[ $blog_id ] = array();
			} else {
				$converted = array();
				$themes    = wp_get_themes();
				foreach ( $themes as $stylesheet => $theme_data ) {
					if ( isset( $allowed_themes[ $blog_id ][ $theme_data->get( 'Name' ) ] ) ) {
						$converted[ $stylesheet ] = true;
					}
				}
				$allowed_themes[ $blog_id ] = $converted;
			}
			// Set the option so we never have to go through this pain again.
			if ( is_admin() && $allowed_themes[ $blog_id ] ) {
				if ( $current ) {
					update_option( 'allowedthemes', $allowed_themes[ $blog_id ] );
					delete_option( 'allowed_themes' );
				} else {
					switch_to_blog( $blog_id );
					update_option( 'allowedthemes', $allowed_themes[ $blog_id ] );
					delete_option( 'allowed_themes' );
					restore_current_blog();
				}
			}
		}

		/** This filter is documented in wp-includes/class-wp-theme.php */
		return (array) apply_filters( 'site_allowed_themes', $allowed_themes[ $blog_id ], $blog_id );
	}

	/**
	 * Enables a theme for all sites on the current network.
	 *
	 * @since 4.6.0
	 *
	 * @param string|string[] $stylesheets Stylesheet name or array of stylesheet names.
	 */
	public static function network_enable_theme( $stylesheets ) {
		if ( ! is_multisite() ) {
			return;
		}

		if ( ! is_array( $stylesheets ) ) {
			$stylesheets = array( $stylesheets );
		}

		$allowed_themes = get_site_option( 'allowedthemes' );
		foreach ( $stylesheets as $stylesheet ) {
			$allowed_themes[ $stylesheet ] = true;
		}

		update_site_option( 'allowedthemes', $allowed_themes );
	}

	/**
	 * Disables a theme for all sites on the current network.
	 *
	 * @since 4.6.0
	 *
	 * @param string|string[] $stylesheets Stylesheet name or array of stylesheet names.
	 */
	public static function network_disable_theme( $stylesheets ) {
		if ( ! is_multisite() ) {
			return;
		}

		if ( ! is_array( $stylesheets ) ) {
			$stylesheets = array( $stylesheets );
		}

		$allowed_themes = get_site_option( 'allowedthemes' );
		foreach ( $stylesheets as $stylesheet ) {
			if ( isset( $allowed_themes[ $stylesheet ] ) ) {
				unset( $allowed_themes[ $stylesheet ] );
			}
		}

		update_site_option( 'allowedthemes', $allowed_themes );
	}

	/**
	 * Sorts themes by name.
	 *
	 * @since 3.4.0
	 *
	 * @param WP_Theme[] $themes Array of theme objects to sort (passed by reference).
	 */
	public static function sort_by_name( &$themes ) {
		if ( 0 === strpos( get_user_locale(), 'en_' ) ) {
			uasort( $themes, array( 'WP_Theme', '_name_sort' ) );
		} else {
			foreach ( $themes as $key => $theme ) {
				$theme->translate_header( 'Name', $theme->headers['Name'] );
			}
			uasort( $themes, array( 'WP_Theme', '_name_sort_i18n' ) );
		}
	}

	/**
	 * Callback function for usort() to naturally sort themes by name.
	 *
	 * Accesses the Name header directly from the class for maximum speed.
	 * Would choke on HTML but we don't care enough to slow it down with strip_tags().
	 *
	 * @since 3.4.0
	 *
	 * @param WP_Theme $a First theme.
	 * @param WP_Theme $b Second theme.
	 * @return int Negative if `$a` falls lower in the natural order than `$b`. Zero if they fall equally.
	 *             Greater than 0 if `$a` falls higher in the natural order than `$b`. Used with usort().
	 */
	private static function _name_sort( $a, $b ) {
		return strnatcasecmp( $a->headers['Name'], $b->headers['Name'] );
	}

	/**
	 * Callback function for usort() to naturally sort themes by translated name.
	 *
	 * @since 3.4.0
	 *
	 * @param WP_Theme $a First theme.
	 * @param WP_Theme $b Second theme.
	 * @return int Negative if `$a` falls lower in the natural order than `$b`. Zero if they fall equally.
	 *             Greater than 0 if `$a` falls higher in the natural order than `$b`. Used with usort().
	 */
	private static function _name_sort_i18n( $a, $b ) {
		return strnatcasecmp( $a->name_translated, $b->name_translated );
	}

	private static function _check_headers_property_has_correct_type( $headers ) {
		if ( ! is_array( $headers ) ) {
			return false;
		}
		foreach ( $headers as $key => $value ) {
			if ( ! is_string( $key ) || ! is_string( $value ) ) {
				return false;
			}
		}
		return true;
	}
}