Well basically all I did was make the overall colors of the suits and other accessories darker. I also added the arm band because, while the suits are darker, some people might find it difficult to see an opponent, so the band should make them stand out more. The tie is also brighter.
Just install the two files into this folder:
C:\Program Files\Steam\SteamApps\Username\team fortress 2\tf\materials\models\player\spy
If any of the folders do not yet exist, make them yourself.
Enjoy it, seriously :D