Well, you can't (I don't think) because when the CSS template is rendered it's running the css.php script and the forum isn't available. You may be able to edit the headinclude_bottom template to include different CSS files depending on the forumid (although I'm not sure what variable you'd check).
Or maybe you could put your CSS in <style type="text/css">...</style> tags in the template you're interested in changing (like SHOWTHREAD).
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