For screen display, Thesis uses a layout based on the em unit. One em is a relative unit, its size or dimensions determined in proportion to other elements on the page.

In this text input, you may assign an "em" value of separation, giving space -- light green at left -- to let your design breathe, moving away from the top bar of the browser window. This space is filled by the body background color or image, depending.

In the image at left, one em of space has been assigned, allowing 1.0 em of a presumed body.custom green to shine through. A stair-step progression of roughly 1-10 ems is shown along the top... this may help you determine the amount of space you add, if any. Em units can be decimals: values could be whole numbers -- or 0.25, 1.2, 3.328...