It depends on what theme you want to port, and how closely you want it to resemble wordpress. If you only want matching colours, then you would only need to modify styles/prosilver/theme/colours.css. If you want something that "matches more", you would be looking at changing other CSS files and probably the html files in /styles/prosilver/template/ as well.
A Donut's Blog
"Bach's Prelude (Cello Suite No. 1) is driving Indiana country roads in Autumn" - Ann Kish