Is this for the main Q2A stylesheet, or for some custom styles? I don't think adding preload makes any difference for the main stylesheet - you want to load it as soon as you get to it, not asynchronously.
But it can be done in your theme. Probably the best function to override would be head_title() as that comes near the beginning of <head>
public function head_title()
$this->output('<link rel="preload" href="stylesheet.css" as="style" onload="this.rel=\'stylesheet\'">');