Navigation bar overlapping to my Caggories

I'm trying to put layers like z-index but it's not working here's a picture what I'm having problem with



You see the green nav and the Home in the left that's my Navigation bar and its position: fixed but when I scroll down the Caggories where you see the topic and stuff, Can someone help me on this.

Link of your forum?

Ok here: http://webgraphics.forumotion.com/

I did the navigation on my own with css but I just can't get so look at it for me

BUMP

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@Theme:
That's strange :O Don't really know what problem at this , sorry.