If you download a theme that is not in a repository you’ll have to download and install it manually. Here’s how:

1. Download the zip file to your Mac/PC.
2. Unzip the archive.
3. Upload the entire “theme” folder to your iPod touch’s either one of the following folders depending on your firmware version:
   • Firmware Ver 1.1.1 & 1.1.2
     /private/var/root/Library/SummerBoard/Themes/
   • Firmware 1.1.3
     /private/var/mobile/Library/SummerBoard/Themes/
4. Launch “SMBpref” and turn ON  the following options:
   • Theme Wallpaper - ON
   • Theme Status Bar - ON
   • Theme Icons - ON
   • Theme Dock - ON
   • Wallpaper - ON
   • Skip Last Row - ON (for firmware 1.1.1 & 1.1.2 only)
5. Look for the Name of the theme you uploaded under “Themes”, and select it.
6. Tap “Restart Springboard”.

Voila you should be looking at your new theme

To make your own SummerBoard theme follow these steps:

1.  Create a new folder with the name of your theme e.g <Example>

2. You need to create 3 images for your theme:

• StatusBar (optional, 320×20 PNG, alpha)
• Wallpaper (320×480 PNG, no alpha)
• Dock (320×91 PNG, alpha)

(alpha = Transparency)

These need to named in the following format:

• Music.png
• Photos.png
• Videos.png

etc.

TIP*

If your using Photoshop use the “Save for Web” option (Shortcut keys: CTRL + SHIFT + ALT + S)  This will reduce and optimize your icons for your ipod touch.

Source: http://www.apptapp.com/