This is how to have fixed Topbar navigation bar in Q2A version: 1.7.5
Add the following in qa-theme/SnowFlat/qa-styles.css
#qam-topbar {
position: fixed;
top: 0;
right: 0;
left: 0;
z-index: 0;
}
.qa-body-wrapper {
margin-top: 100px;
}
Then edit qa-theme/SnowFlat/qa-theme.php Line 44
private $fixed_topbar = true;