How to set max width for content?

Hi, I have just bought the Pro version of Adventurous theme. It is great so far (even though I am not sure about the soundness of my investment - free version might have sufficed. But anyways..) I use the no-column option (www.coursdeguitarenancy.com). I would like the only column not to stretch accross the whole screen. Instead I would like it to be, say, 980 pixels wide maximum on wide screens. And still be mobile friendly. I searched the forum and could not find a way to do this. If such a request was made before and you gave a solution, can you please point me to it? Thanks, Frédéric