there are several ways of doing this.
1. You can use the theme-it module
2. Go to the folder css menuwriter and in the sub directories you will find all the css files for both admin and public areas
3. Open in dreamweaver highlight the menu item and click in the wagon wheel that appears (cant think of the name)
That will reveal the code navigator which lists all the css items relating to that item.
Using that you can work out what rule to change
This can get quite complicated with the hover, active, visited rules etc so take a backup before you start and then as you suceed in one change take further backup.