Removing the sidebar from all posts

I love the look of this theme but I’m struggling with some of the customisation.

I want to remove the sidebar from my posts, to make them full page width. But I can’t work out how to do it. All I’ve managed to do is remove all the content from the sidebar. Now I have a white space all down the right hand side of my post pages.

I’m definitely not a web designer or techie of any sort so please be gentle!