With what help to create such a design?

Question

css

In the image below you can see what I want to do but don't know how.
Screenshot taken here: demo.joomla.org
This is the effect I want to apply in the design of the site.
Please, tell me how can I implement this, provided that it will go to the design of the site? CSS3?

Answer 1

Repeating jagged background
This is if the old IE is not needed.
Otherwise, a picture. Less hassle.

Answer 2

demo.joomla.org/templates/joomla/images/demo/modBox.png is made on the example as a picture