I think it would be a good idea to make the CSS to use selectable, so one has not to nullify existing settings before applying own ones. I just created a patch for this: it adds a new option to the [menu-custom] section of the trac.ini - menu_style. Here one can either select from the "shipped" CSS files (with my patch: 'simple' (the current one), 'superfish' (the one used previously), and 'trac' - the one I made), or define a complete URL for an own CSS file (which then is taken literally).
[menu-custom]
menu_style = superfish
For this to work, the current superfish.css file has to be split to basic.css (holding the "essential" part) and simple.css (holding the "Demo Style"). Plus from the previous superfish.css, the "Demo Style" part should be added as superfish.css. If you lost my trac.css, I'll attach it again here (wait - I'll either attach it again later, there are some modifications I need to finish).
Hope you like and apply the patch ;)
All the best,
Izzy.