If you have an existing theme you wish to retain for your website, you could always install JobRoller in a subdirectory, e.g:
yourdomain.com <== Your existing homepage
yourdomain.com/jobs <== Your JobRoller installation
You could then restyle JobRoller to match your existing homepage, giving the site visitor a more seamless experience. The theme is not designed to be used inside of another theme in the way that a plugin is. Thanks.