If you want all posts to show full page width there is no need to do any coding. In the Dashboard under Graphene theme options, select the display tab, then choose column options.
But, I see you probably do not want to lose all of the sidebar. So, that will not work for you.
Sorry, I can’t tell you where to find the place to make the change. But, I will make one comment from a readability perspective. I am not fond of sites that make the text run the full width of the screen. I find it tiring to try to read lines of text that are that wide. I usually try to narrow the width of the display in hopes that the screen is a flexible width.
Your site looks very nice.
Good luck.