How to remove the right sidebar space ?

Hi,

I really like this theme, but mostly in its “responsive” form, when the content is centered and there isn’t any sidebar.

I know responsive CSS can’t be modified easily, so I’m only asking : how to get rid of this sidebar, since I don’t use it ?

I am not afraid of getting my hands dirty, I already created a child theme.

Thanks a lot!

The page I need help with: [log in to see the link]