The simplest solution would be to put this CSS somewhere in the given question:
<style>
.header2 a {
color: #FF0000;
}
</style>
Alternatively, if you want to keep all the CSS in the head tag, you could modify your question HTML to be something like this:
<div class="popup">[% PopUp("Link", "file.htm", 500, 500) %]</div>
And then put this code in your head tag after the CSS you posted above:
.popup a {
color: #FF0000;
}