Is there any way to adjust the side margins in Reading Mode? For instance, a slider that adjusts the width of the text? There is a feature like this in the OS X Kindle app. I've included the screenshot.

I don't know about anyone else, but I'd use Reading Mode a lot more for leisurely, distraction-free reading if the text wasn't so wide! Right now, I don't use it at all because it hurts my head. I know that I can make the text one column and it adds a little more padding on the side, but it would be nice if I could adjust it to my liking!