GMail also has developed a number of new tools which can be used to alter your background image. In addition to toggling from light to dark text for your inbox, you can do a blurring or vignette effect which makes the image darker moving from the corners of the image inward. These tools have sliders which increase or decrease the amount of blur. In the image below, I highlighted in yellow the two tools - vignette and blurring effect.

The Vignette tool is the fist one which looks like an oval inside a square. To understand better what vignette editors do, you can view the two images of Ulysses S. Grant. The first one is the original and the second one has been created using a vignette editor.

The blurring effect is the second one which looks like a series of dots. To see the effects of the blur tool, note the two images of Ulysses S. Grant below. The first one is the original and the second one has been used with the blurring effect.