The Tufte-Jekyll theme is a natural extension of the work done by Edward Tufte and his collaborators on Github who created a CSS file that allows web writers to use the same simple and elegant style employed in his published materials.
To incorporate these styles into a Jekyll theme, I have made some very slight modifications that attempt to maintain the feel of the CSS styles in his Github repo.
Note that this is a full-width layout. This was accomplished by including
layout: full-width in the YAML front matter for this page. Keep in mind that all the Tufte-Jekyll sidenote and marginnote goodness will not work on full-width layouts!
You can find the source code for Jekyll at github.com/jekyll/jekyll.