Hi,
We are having strong experience on integration of WHMCS and editing the existing theme or designing custom theme and integrating into WHMCS.
The back end of WHMCS is stay same as how whmcs is provided but we can make adjustments and new design of front end. I hope you want it to be done for your web hosting, dedicated servers business.
Let me know if you have any question.
Thanks & Regards,
Yogendra