Assuming you're talking about the SnowFlat theme, what you'd need to do is add a margin to the qa-body-wrapper class. I think this should work:
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
Then edit qa-theme/SnowFlat/qa-theme.php Line 44
private $fixed_topbar = true;
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