How to reduce background in small devices?

Hi everybody,

I have been working on a child theme of twentyeleven for some months and I’m quite happy with the results, except for one small thing: a grey zone appears in the right side of my template’s background, in small screens like tablets and mobile phones. This problem is not visible on my computer, and I’m unable to identify what element I have to resize, because I used to work with firebug on firefox, and I don’t know how to work on my mobile phone.

If the problem is not clear, you maybe could take a look on my site with a tablet or a mobile phone. So you will see that you can move the screen to the right, and then a fragment of the grey background appears. The problem doesn’t exists on the left side of the template, which works fine.

Hope this description is clear and someone can help me.