Theme not working when using non-standard port number
It should just be a matter of setting the $base_url in settings.php


  If Drupal is generating incorrect URLs on your site, which could be in HTML headers (links to CSS and JS files) or visible links on pages (such as in menus), uncomment the Base URL statement below (remove the leading hash sign) and fill in the absolute URL to your Drupal installation.


For your case that would be:

$base_url = 'http://mysite.local:89';