Body padding should equal fixed top bar height plus margin-bottom.

When wrapping a top-bar in `<div class="fixed">`, the body's padding-top is set to the top-bar's outer height. However, this actually pushes content farther up, leaving less room between the content and the top-bar than when it's not fixed, due to the top-bar's `margin-bottom: 1.875em;`.

This patch adds the height and the bottom margin and sets the body's padding-top equal to the sum, meaning the content layout stays consistent.