@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
{php}
define('WP_USE_THEMES', FALSE);
require('/path/to/wordpress/wp-load.php');
{/php}
Next, call WordPress Header functions into your header.tpl
{php}
get_header();
{/php}
Next, call WordPress Footer functions into your footer.tpl
{php}
get_footer();
{/php}
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.