QUOTE(shoutout @ Dec 25 2007, 12:20 AM)
After you download your  theme  file from the internet, here's how to transfer it to your PC. 

Install the Nokia PC suite from the CD that you got with your phone.

Connect your  Phone to your PC using a compatible data cable or Bluetooth  (If your PC is Bluetooth compatible).
After your phone sync's up with your pc, open My Computer.

Double-click on Nokia Phone Browser

Now you should be able to see what's stored in your phone memory and on the memory card.

Open the source where you stored your themes file.

Copy them, and paste it to Memory Card > Themes directory of your phones memory card.
On your phone, navigate to Settings and you will find the copied themes. 
Navigate to the file and select the theme and select Apply.