They tend to look for a portfolio showing a number of different sites, each showing a variety of skills in various areas. So if all your sites were built on WP templates, there would be little evidence of your ability to code html from scratch. A variety of layouts, good examples of design skills and use of colours, a few special effects (say some jQuery), some simple but good looking sites, perhaps some graphically complex sites. Perhaps some testimonials from satisfied customers. Do you create just html pages, or can you also do database driven sites? Do your sites always show the same special effects every time, or are a range of effects used, to suit each client's site?
These days most professional design firms have workflow down to a science. Usually you have a designer who can put together a layout and hand a finished .psd to the next guy to cut it up into an html template. That gets passed to the programmer(s) who add any necessary backend functionality (database, scripting, etc).
Most firms stick with a standard CMS such as Wordpress, Drupal, Joomla, Viper and/or forum software such as vBulletin, SMF, Invision or Insight.
If I were trying to put together a portfolio for employment, I'd definitely put together some templates for the CMS packages mentioned above to show you have knowledge of the packages.
btw, excellent design on the site you linked to. I'd be interested in seeing some more of your work.