I want to set global directory locations for my CSS and JS locations. How?

Sure you can do that. Create a simple plugin that looks something like this:

<?php
/*
Plugin Name: SiteWide Assets Location
Plugin URI: www.mysite.com
Description: Global URI for commonly used CSS and JS files
Version: 1.0
Author: Me
License: None
*/
define( 'SYSTEM_ASSETS', $_SERVER['DOCUMENT_ROOT'] . '/somedirectory/' );

Then, store your .css and .js files there (or in a subdirectory within that directory), and now in your themes you can simply refer to the constant SYSTEM_ASSETS whenever you want that URI location.