Like David, I used TwentyFourteen as the base for my theme, and have been editing it to suit my needs. One of the things that bothered me was the excessive padding around the content area. As David points out in his post, its not so much a padding issue as it is the max-width set in the css file.
Two things I did differently.
First off, I did not edit the TwentyFourteen theme directly. Instead I am using a child-theme with TwentyFourteen as the template, this is the way that WordPress recommends to prevent overwrites when theme updates happen.
The second thing was the content width on all pages/posts/etc. David’s post only covers changing the page-content class, which will only change the width on actual WordPress pages. Instead, in my child-theme I did the following:
Since this was in my child-themes css file, it will override the max-width on all posts instead of just pages. I did not have to include the margin entry either since that is already set in TwentyFourteens css file.