WordPress at a glance
function is not described

wp_update_themes() WP 2.7.0

Check theme versions against the latest versions hosted on WordPress.org.

A list of all themes installed in sent to WP. Checks against the WordPress server at api.wordpress.org. Will only check if WordPress isn't installing.

Hooks in function
Return

Nothing (null).

Usage

wp_update_themes( $extra_stats );
$extra_stats(array)
Extra statistics to report to the WordPress.org API.
Default: array()

Code of wp update themes: wp-includes/update.php WP 5.2.1

<?php
function wp_update_themes( $extra_stats = array() ) {
	if ( wp_installing() ) {
		return;
	}

	// include an unmodified $wp_version
	include( ABSPATH . WPINC . '/version.php' );

	$installed_themes = wp_get_themes();
	$translations     = wp_get_installed_translations( 'themes' );

	$last_update = get_site_transient( 'update_themes' );
	if ( ! is_object( $last_update ) ) {
		$last_update = new stdClass;
	}

	$themes = $checked = $request = array();

	// Put slug of current theme into request.
	$request['active'] = get_option( 'stylesheet' );

	foreach ( $installed_themes as $theme ) {
		$checked[ $theme->get_stylesheet() ] = $theme->get( 'Version' );

		$themes[ $theme->get_stylesheet() ] = array(
			'Name'       => $theme->get( 'Name' ),
			'Title'      => $theme->get( 'Name' ),
			'Version'    => $theme->get( 'Version' ),
			'Author'     => $theme->get( 'Author' ),
			'Author URI' => $theme->get( 'AuthorURI' ),
			'Template'   => $theme->get_template(),
			'Stylesheet' => $theme->get_stylesheet(),
		);
	}

	$doing_cron = wp_doing_cron();

	// Check for update on a different schedule, depending on the page.
	switch ( current_filter() ) {
		case 'upgrader_process_complete':
			$timeout = 0;
			break;
		case 'load-update-core.php':
			$timeout = MINUTE_IN_SECONDS;
			break;
		case 'load-themes.php':
		case 'load-update.php':
			$timeout = HOUR_IN_SECONDS;
			break;
		default:
			if ( $doing_cron ) {
				$timeout = 2 * HOUR_IN_SECONDS;
			} else {
				$timeout = 12 * HOUR_IN_SECONDS;
			}
	}

	$time_not_changed = isset( $last_update->last_checked ) && $timeout > ( time() - $last_update->last_checked );

	if ( $time_not_changed && ! $extra_stats ) {
		$theme_changed = false;
		foreach ( $checked as $slug => $v ) {
			if ( ! isset( $last_update->checked[ $slug ] ) || strval( $last_update->checked[ $slug ] ) !== strval( $v ) ) {
				$theme_changed = true;
			}
		}

		if ( isset( $last_update->response ) && is_array( $last_update->response ) ) {
			foreach ( $last_update->response as $slug => $update_details ) {
				if ( ! isset( $checked[ $slug ] ) ) {
					$theme_changed = true;
					break;
				}
			}
		}

		// Bail if we've checked recently and if nothing has changed
		if ( ! $theme_changed ) {
			return;
		}
	}

	// Update last_checked for current to prevent multiple blocking requests if request hangs
	$last_update->last_checked = time();
	set_site_transient( 'update_themes', $last_update );

	$request['themes'] = $themes;

	$locales = array_values( get_available_languages() );

	/**
	 * Filters the locales requested for theme translations.
	 *
	 * @since 3.7.0
	 * @since 4.5.0 The default value of the `$locales` parameter changed to include all locales.
	 *
	 * @param array $locales Theme locales. Default is all available locales of the site.
	 */
	$locales = apply_filters( 'themes_update_check_locales', $locales );
	$locales = array_unique( $locales );

	if ( $doing_cron ) {
		$timeout = 30;
	} else {
		// Three seconds, plus one extra second for every 10 themes
		$timeout = 3 + (int) ( count( $themes ) / 10 );
	}

	$options = array(
		'timeout'    => $timeout,
		'body'       => array(
			'themes'       => wp_json_encode( $request ),
			'translations' => wp_json_encode( $translations ),
			'locale'       => wp_json_encode( $locales ),
		),
		'user-agent' => 'WordPress/' . $wp_version . '; ' . home_url( '/' ),
	);

	if ( $extra_stats ) {
		$options['body']['update_stats'] = wp_json_encode( $extra_stats );
	}

	$url = $http_url = 'http://api.wordpress.org/themes/update-check/1.1/';
	if ( $ssl = wp_http_supports( array( 'ssl' ) ) ) {
		$url = set_url_scheme( $url, 'https' );
	}

	$raw_response = wp_remote_post( $url, $options );
	if ( $ssl && is_wp_error( $raw_response ) ) {
		trigger_error(
			sprintf(
				/* translators: %s: support forums URL */
				__( 'An unexpected error occurred. Something may be wrong with WordPress.org or this server&#8217;s configuration. If you continue to have problems, please try the <a href="%s">support forums</a>.' ),
				__( 'https://wordpress.org/support/' )
			) . ' ' . __( '(WordPress could not establish a secure connection to WordPress.org. Please contact your server administrator.)' ),
			headers_sent() || WP_DEBUG ? E_USER_WARNING : E_USER_NOTICE
		);
		$raw_response = wp_remote_post( $http_url, $options );
	}

	if ( is_wp_error( $raw_response ) || 200 != wp_remote_retrieve_response_code( $raw_response ) ) {
		return;
	}

	$new_update               = new stdClass;
	$new_update->last_checked = time();
	$new_update->checked      = $checked;

	$response = json_decode( wp_remote_retrieve_body( $raw_response ), true );

	if ( is_array( $response ) ) {
		$new_update->response     = $response['themes'];
		$new_update->translations = $response['translations'];
	}

	set_site_transient( 'update_themes', $new_update );
}