Hi Marksu,

today is your lucky day...I also just spent like hopurs on the same issue, decided to post as you can see above and few minutes ago I got the solution....even didnt have time to post an update...

go into your stylesheet and find
div#nav-cat ul {margin: 0; padding: 0; padding-top: 0; width: 150px; background-color: transparent;
border: 2px solid tan;
background-repeat: no-repeat;
}
should be at the beginning line 11

change background-color: transparent
to #ffffff or which color you want...

regards,
kruna