I think the sfWebDebug css class currently named "top" should be renamed to something more specific to sfWebDebug, say 'sfWebDebugTop'. I have a stylesheet that I inherited that also uses a "top" class, leading to issues both with the debug toolbar and the layout div from conflicting styles.
Reproducing is easy:
1. Create a page with the web debug toolbar enabled.
2. Add a div to that page with the class "top".
3. Set the css style for that new div to height:10px; Other styles to try are setting the background color to light gray, text color to white, etc.
4. Poof! Can't see anything on the web debug toolbar.
I tried to make a patch, but somehow ended up rendering the sfWebDebug divs up at the top unsee-able. :-D