How to make website container bigger?

Hello,
I been advice to use a new them because my other one was slowing my website. On the new them "Klein" the container area looks small and I don't like it (http://positifstyle.com/actualites/)
I tried different css codes I find in blogs but it didn't work. Can any one has an ice of what to do? Its not in the them setups that for sure!
Thank you in advance!
Regards,