Threads use the SHOWTHREAD template. Just before </head> you could add the following condition:
HTML Code:
<vb:if condition="$thread['postuserid'] == 1">
{vb:cssfile davethreadstyle.css}
</vb:if>
Then create a new template called davethreadstyle.css (or whatever you named it in the code above) and it will load it in a thread when the thread is started by userid 1. Add your custom CSS to the CSS template.
Alternatively, instead of injecting a CSS file, you could use the regular CSS style tag like this:
HTML Code:
<vb:if condition="$thread['postuserid'] == 1">
<style type="text/css">
.element{
attribute: value;
}
</style>
</vb:if>