Changing theme width

Hi everyone,

I posted a message in theme topic, but maybe you don't read it often, so I'll write here.
My original message text is:

"mmm, can anyone tell me how to make notebook theme less wide? On owner screenshots it looks ok, but on my site is completely streched around screen (from side to side), and it look so huge.
I was looking in .css file but don't know what exactly to change to make whole theme less wide. I'm not so good with programming..

Any help would be very appreciated.