This is not so much a SilverStripe question as a general question on frontend design. There are no such things as the 'uneven boundaries' you're looking for in HTML/css - this is an illusion created by using (background) images that suggest an uneven line.
If you want to experiment, I suggest playing around with the existing Drupal or Silverstripe theme(s) adding interesting background images and see what happens. But I wouldn't bas the choice of my CMS on the use of certain backend images in a theme, if I were you :)
Martine