In functions.php
you can create a constant with the user id, like this.
// if somehow this constant exists this will prevent a php error
if (!defined('CURRENT_USER_ID')) {
// if user is not logged it this constant will contain 0
define('CURRENT_USER_ID', get_current_user_id());
}
Now you have a constant variable available anywhere in your theme.
To access it you can simply call it, for example
echo CURRENT_USER_ID;