Any decent CMS is working with themes/templates and is totally flexible when it comes to implementing your own design. Of course, your design has to be flexible to adapt to dynamic content and if it isn’t then it’s not a good design in the first place, this is my honest opinion.
So, try out a few CMSs on http://www.opensourcecms.com/
and choose the one you think is most suitable for your client. I had/have good experiences with CMS Made Simple and Concrete5. Wordpress is OK, too, but for me (still) more a blogging software rather than a real website CMS.