> The body max-width is 800px by default. Research has shown that limiting the width can lead to better retention of the content itself, as well as a decrease in eye strain. A thinner column of text is more readable on very large monitors. This is simple to change by editing the max-width of the body element in your custom.css file.

No, edit the max-width of your _content container_. Not every pixel of a site is devoted to content worth retaining.