4
Your theme file has "localhost" . If you instead had something like <{$xoops_url}>/xoops.css and <{$xoops_themecss}> it would work better and follow any DNS changes (from the mainfile.php file).
Also, the statements should prefix with the theme folder name.
Your mainfile.php file needs to reflect the actual hostname being used. For running from the inside (behind firewall) you need some DNS tricks or firewall tricks. Could also simply have a hosts file pointing to the internal IP.
Either way, you want to run it with the real hostname and not use "localhost".