Removing the fancy lighting effect in beamer enumerate bullets

Beamer bullets using enumerate have this cutesy lighting effect that I guess is meant to make the bullets seem 3D. In fact, they are simply hard to read, as the pic below illustrates.

Is there any way to make the bullets have a flat background color, i.e., removing the "lighting" effect? I scoured the manual for this, but I don't really know what a good keyword would be. Thanks!