Twenty Fourteen theme page area

I’m using this theme to replicate a website – but is there anyway I can adjust the margins in the main page content area. They’re about 150 pixels on each side, which squeezes the text into a long narrow strip (and there’s a lot of text on this site).
I have already changed the side bar from black to green, but stuck on how to tweak the page attributes.
The website is https://www.hothamparkheritagetrust.co.uk/diary/
I would welcome any suggestions here, as I’m fairly new to wordpress.