Hi heeseung,
First of all, this is a WordPress Theme. Moreover, we provided WHMCS template to use separately with the latest WHMCS support and our provided WHMCS template is fully compatible with latest stable WHMCS v7.5.1.
Furthermore, we are not providing any backdated WHMCS support like WHMCS 7.4.2. Because there was a lot of bugs that fixed by WHMCS team in the latest WHMCS version. Also, they added more features in the latest WHMCS version.
Best Regards,
ThemeLooks Support Team