How to remove that blue thing from my navigation ?

I think about this blue background
http://img40.imagefra.me/img/img40/2/1/16/naca007/f_14htlt2oojam_48df805.png

Hi,

Are you meaning the advertising it's below the navbar ?
If yes, for the members you can disable them in the AP>> General >> Configuration > Activate ads : > Guest

No , I mean this blue background below navigations

http://img38.imagefra.me/img/img38/2/1/16/naca007/f_14htlt2oojam_55b91f9.png

Han ok !

in which version is the foum concerned ?

punbb

Ouch, punBB ?!?

Try with this code in your css stylesheet :

#pun-head #pun-navlinks {background-color: #transparent; border-color: #transparent;}

It works . Thank you very much for help ^.^