Source

File: wp-includes/theme.php

function get_stylesheet_directory() {
	$stylesheet     = get_stylesheet();
	$theme_root     = get_theme_root( $stylesheet );
	$stylesheet_dir = "$theme_root/$stylesheet";

	/**
	 * Filters the stylesheet directory path for current theme.
	 *
	 * @since 1.5.0
	 *
	 * @param string $stylesheet_dir Absolute path to the current theme.
	 * @param string $stylesheet     Directory name of the current theme.
	 * @param string $theme_root     Absolute path to themes directory.
	 */
	return apply_filters( 'stylesheet_directory', $stylesheet_dir, $stylesheet, $theme_root );
}