In unity-control-center and unity-tweak-tool I did not find this option. MyUnity is not avaliable anymore.
If there is a way to get rid of all panel/window (semi)transparencies in Ubuntu, it would be even better.
There doesn't seem to be a direct option. Only way I could get around was to change the background color. Using tweak, under launcher, appearance, change the background color to a darker color (I used black) and then play with the transparency level to adjust the contrast.