Fix small issue with gritter-light.png

There was an area in this image where it looked like the non-alpha values of the pixels were inverted - the whites should have been blacks and the blacks should have been whites. This was causing the close icon to look slightly wrong. If you open the images up in the GIMP and restrict the alpha values to 255 using the "Levels" tool, you'll be able to more clearly see the difference between the two images. I didn't change the alpha channel at all.

Thanks!