Sitebuilder Navigation bar to be fixed??

[Asama]

Hello everybody,
I'm making the header of my forums fixed and i don't want any margin on the top and i want the header be on the top of everything so on scroll nothing can cover or hide it and for that i'm just done using z-index but the problem is that it's hiding completely the sitebuilder navigation bar.
So i tried editing something in css_additional.css picking up the classes names from the sitebuilder.css but nothing seems to work because it's always hided. So i thought to make the header top margin to 26px so it shows up the sitebuilder navigation without hiding it but the problem is when i scroll the page, the sitebuilder get scrolled so there's a gap that i don't want!
Can anybody tell me how to increase the sitebuilder z-index or just how to make it fixed?????
Here's my forums: https://forum.xtreme-developing.com
Thanks in advance.