The main tweak I made to this theme when I downloaded it was that I made the main column more narrow than the original designer. Let me explain why.
Have you ever noticed that newspapers and magazines use narrow columns? There’s a reason for that, and those lessons should be applied when displaying text on the web.
Narrow columns are faster to read because the reader’s eyes don’t have to scan as far back-and-forth, and can rely on peripheral vision to take in words at the beginning and end of the lines.
In addition, narrow columns are easier on the eyes, so the reader can read longer before tiring. Each time the reader’s eyes perform a “carriage return”–moving from the end of a line and move back to the beginning of the next–they get a little rest.
The net result is text that is both faster to read and easier on the eyes.
P.S. Tangentially related, when I was trying out the iPhone Kindle recently, one of the thing that made it a real delight to read was that the text is very narrow in on the iPhone. Notwithstanding the brightness of the screen, I was able to read very quickly for a long time without eye fatigue.I really enjoyed that.