Source

File: wp-includes/theme.php

function get_stylesheet_directory_uri() {
	$stylesheet         = str_replace( '%2F', '/', rawurlencode( get_stylesheet() ) );
	$theme_root_uri     = get_theme_root_uri( $stylesheet );
	$stylesheet_dir_uri = "$theme_root_uri/$stylesheet";

	/**
	 * Filters the stylesheet directory URI.
	 *
	 * @since 1.5.0
	 *
	 * @param string $stylesheet_dir_uri Stylesheet directory URI.
	 * @param string $stylesheet         Name of the activated theme's directory.
	 * @param string $theme_root_uri     Themes root URI.
	 */
	return apply_filters( 'stylesheet_directory_uri', $stylesheet_dir_uri, $stylesheet, $theme_root_uri );
}