@adamjedgar it would be my recommendation to simply use WHMCS service to create you a matching theme for your Wordpress website. You can order this solution here https://www.whmcs.com/members/cart.php?a=confproduct&i=0
I do also understand if you want to achieve this on your own. You can fairly easily integrate your Wordpress header and footer into your whims template by using the following method. ( This is what I did )
First step is to bring in the WordPress core into your header.tpl
Next, call WordPress Header functions into your header.tpl
Next, call WordPress Footer functions into your footer.tpl
As far as the rest of the WHMCS styling, buttons etc you will need to do some css changes to reflect your Wordpress design.
This topic has been discuss a number times and the common consensus is to still use a professional service like the one I mentioned above.
Credits for this solution are from here https://squidix.com/blog/include-wordpress-header-footer-whmcs/
As I said, this is how I have integrated my main sites design into whims and I'm happy to help where I can.