Themes are usually stand-alone installation files.
I got my themes from these places:

http://thememaker.garage.maemo.org/Themes.html
http://maemo.org/downloads/product/1...os2006-themes/

Simply download them directly to you 770 or download to you desktop and then transfer them with the USB cable.
Once the .deb files are in your 770, simply double tap to install..

hope this helped.