It is very easy. Download the firmware package for a 77xx (e.g. Rogers) and install this package to your computer. Then goto C:\Program Files\Common Files\Research In Motion\Shared\Loader Files\7700-v18.104.22.168_P22.214.171.124\Java and copy the file net_rim_theme_blackberry_large.cod to separate directory. Now use javaloader.exe to upload the file to your Blackberry with the following command line:
javaloader -usb load net_rim_theme_blackberry_large.cod
Afterwards goto Options->Themes on your Blackberry and change to the new theme.
If you do not have the javaloader.exe than you can use Application Loader from Desktop Manager to load the new theme to your Blackberry. To do this you must create a simple text file with the extension alx (e.g. theme.alx) which contains the following text:
<application id="BB 7730 Theme">
BB 7730 Theme
The cod file and the alx file must be located in the same directory. After starting Application Loader browse to this directory and add it to the app list.