SMF Version : 2.0.5
Theme: Vivanto
Problem: When I set position: fixed in css for div topnav, when skroll half of toolbar dissapears, it is transparent + wrapper it be below toolbar .. When I delete position: fixed, then it works!
Picture:

Thanks a lot!
For theme Related Questions it is best to ask the theme author. You can find a link to a topic for the theme or a support board on the theme's page in the Theme Site (http://custom.simplemachines.org/theme/)
This theme is a paid-for theme on DzinerStudio.com, they are the best place to get support... assuming you have a club membership - and if not... well, best not to speak of that really.