I'll try to make this as simple as possible, but it looks like there are a number of changes that need to be made in order to get your desired result.
Starting with the changes that hold the biggest visual effect, you'll need to modify two classes: .site and .hentry
First, with the .site class, you'll just need to remove the background-color that is being set.
Next with the .hentry class you'll need to make a few changes:
margin: 0 auto;
This will get you close, but there are still a few tweaks to make so that everything looks a little better.
Remove the margin-top property from .masthead-fixed .site-main and .article
Remove the padding-top property from .list-view .site-content .hentry:first-of-type, .list-view .site-content .hentry.has-post-thumbnail
That might be it, but I guess it'll depend on what you think once the changes are made. There are some adjustments with padding that could probably be made since a little space was lost.