An external themes directory (above wp root folder) that runs on a separate domain. Best used as mu plugin for your development stack. Greatly simplifies local development. Props: Thomas "toscho" Scholz.

external_themes_dir.php

<?php
/* Plugin Name: Local Theme Roots */

/**
 * Create a custom theme directory on a custom domain.
 * Best used as mu plugin in a local development stack.
 *
 * @author Thomas Scholz
 * @param  string $root URL or path
 * @return string
 */

add_filter( 'theme_root_uri', 'switch_theme_root_local' );
add_filter( 'theme_root',     'switch_theme_root_local' );
function switch_theme_root_local( $root )
{
	// local multi-sites end with '.dev'. Example: wprepo.vanilla-mu.dev
	if ( 0 !== stripos( strrev( $_SERVER['HTTP_HOST'] ), 'ved.' ) )
	{
		return $root;
	}

	if ( 'theme_root_uri' === current_filter() )
	{
		remove_filter( current_filter(), __FUNCTION__ );
		return "http://themes.dev/";
	}

	// If we made it so far we are in the 'theme_root' filter.
	$new_root = trailingslashit( dirname( dirname( __DIR__ ) ) ).'themes.dev';

	# Too early to use register_theme_directory()
	$GLOBALS['wp_theme_directories'][] = $new_root;

	remove_filter( current_filter(), __FUNCTION__ );
	return $new_root;
}