Thanks,
Sorry, I may need a little more help

and for anyone else who may wish to use this.
My board has several styles. To find which one I wish to modify, I go to the admincp and find 'Styles & Templates', then 'Style Manager'. Look for the style you wish to manage, hold you cursor over the style and note the style number in the lower left of your browser window. In my case, 'forums/forums.php/styleid=37'.
Now I start to have a problem.
I can use my html editor and go into my forums/clientscript/vbuletin_css and I find two similar directories 'style00037l' and style00037r', each containing and 'additional.css'.
I may be lost in the wrong area, as they seem to be formatted a little differently that the line you suggest. I tried adding the line to each of them, and it did not seem to work.
Am I in the right place, and just need to poke around a little more?
Thanks