In Suffusion rarely you will need to edit files. Most of changes can be done by the way of Suffusion Options (by eg. for colors you can go at Suffusion Options -> Skinning -> Navigation Bar Above Header and change the colors for menu items there). Explore those options and you will find that almost all features can be changed from there.
For specific requirements, like yours, with a tool like Firebug (Opera, Chrome and Safari have included Inspect Element) you can find the class or the id of desired elements. (see at this recipe of Sayontan how to use those tools: http://aquoid.com/news/2010/02/there-ar ... es-option/
Then, your customised styles will be added at Suffusion Options -> Backend -> Custom Includes -> Custom Styles.
All changes made in options, including those codes added at Custom Includes will be safe in case of upgrades of theme... your modified files will be overwritten.
Now... the id of navigation bar above header is #nav-top
. Use Inspect Elements to see if I'm right