Source

File: wp-includes/theme.php

function get_theme_root_uri( $stylesheet_or_template = '', $theme_root = '' ) {
	global $wp_theme_directories;

	if ( $stylesheet_or_template && ! $theme_root ) {
		$theme_root = get_raw_theme_root( $stylesheet_or_template );
	}

	if ( $stylesheet_or_template && $theme_root ) {
		if ( in_array( $theme_root, (array) $wp_theme_directories, true ) ) {
			// Absolute path. Make an educated guess. YMMV -- but note the filter below.
			if ( 0 === strpos( $theme_root, WP_CONTENT_DIR ) ) {
				$theme_root_uri = content_url( str_replace( WP_CONTENT_DIR, '', $theme_root ) );
			} elseif ( 0 === strpos( $theme_root, ABSPATH ) ) {
				$theme_root_uri = site_url( str_replace( ABSPATH, '', $theme_root ) );
			} elseif ( 0 === strpos( $theme_root, WP_PLUGIN_DIR ) || 0 === strpos( $theme_root, WPMU_PLUGIN_DIR ) ) {
				$theme_root_uri = plugins_url( basename( $theme_root ), $theme_root );
			} else {
				$theme_root_uri = $theme_root;
			}
		} else {
			$theme_root_uri = content_url( $theme_root );
		}
	} else {
		$theme_root_uri = content_url( 'themes' );
	}

	/**
	 * Filters the URI for themes directory.
	 *
	 * @since 1.5.0
	 *
	 * @param string $theme_root_uri         The URI for themes directory.
	 * @param string $siteurl                WordPress web address which is set in General Options.
	 * @param string $stylesheet_or_template The stylesheet or template name of the theme.
	 */
	return apply_filters( 'theme_root_uri', $theme_root_uri, get_option( 'siteurl' ), $stylesheet_or_template );
}