I can't find a way to change the color of the highlighted selection in the autocomplete box in Eclipse.
As you can see from the screenshot white text on light gray background is not very readable.
Ubuntu 11.10 with Unity and Eclipse Classic
Update: This is system dependent, indeed. Changing the Ubuntu theme changes this color. There are settings about this in /usr/share/themes/Ambiance/, but I can't figure out which one. The background is a gradient, so picking the color and grepping for it does not work.