Our themes are stand alone WordPress themes, so therefore require their own instance of WordPress to run. So, no, they're not designed to be integrated straight into an existing theme, but you could of course install WordPress with JobRoller in a subdirectory and customize the theme to "match" like your site it you wanted to.
e.g.
www.yourdomain.com <== Your existing theme
www.yourdomain.com/careers <== Your JobRoller installation