This puts a grey 'glow' behind your navigation links. Put it in your custom css.
nav#main-navigation {
text-shadow: 0px 0px 3px #aaa;
}
The parameters (in order) are
Horizontal shadow offset - currently set to 0px so it sits behind the image)
Vertical shadow offset - currently set to 0px so it sits behind the image)
Blur radius - set to 3px so that it casts an omnidirectional shadow. Reduce this value to make the shadow sharper.
Color - currently set to a mid gray