how to use ToolbarComboPreviewCSS

I'm using FCKeditor to edit a manually selected file, automatically
setting EditorAreaCSS to the corresponding css file.
I want the Style box to show the styles used in the editor area,
so I set ToolbarComboPreviewCSS to point to the same css file.

$oFCKeditor = new FCKeditor('editarea') ;
$oFCKeditor->BasePath = '/cms/fckeditor/' ;
$oFCKConfig->EditorAreaCSS = '../../file1.css' ;
$oFCKConfig->ToolbarComboPreviewCSS = '../../file1.css' ;
$oFCKeditor->Value = $html1 ;
$oFCKeditor->Create() ;

But the ToolbarComboPreviewCSS setting seems to have no effect.
Have I used a wrong syntax, or misunderstood the purpose of this setting?
Please help!

Egbert