[Dave]

Threads use the SHOWTHREAD template. Just before </head> you could add the following condition:

<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:

<vb:if condition="$thread['postuserid'] == 1">
   <style type="text/css">
      .element{
         attribute: value;
      }
   </style>
</vb:if>