You could do this:
<!--[if IE 7]><!-->
<link rel="stylesheet" href="/style/ie7.css" />
<!--<![endif]-->
Notice how the syntax highlighting changes because the link
element is no longer commented out by the conditional statements.
The only caveat is that IE10 and later will also use this stylesheet, because IE10 and later now ignore HTML conditional statements entirely by design. Since IE10 is pretty well-regarded in terms of standards compliance, this should not be a problem, but if you don't want any version of IE except 7 to use this stylesheet, I don't think there's a workaround for it that doesn't involve JavaScript.