Okay here is how to create a basic theme for your iPod.
Things you will need:
1. Jail broken iPod
2. Knowledge on how to SSH
3. An SSH program such as WinSCP
4. Any image software, such as photoshop, microsoft paint etc.
First start off by creating a new folder on your desktop (easy to find) and rename it to whatever you want. Example: My Theme
Inside the folder you will need three images: lock backround, status bar & dock all in the .PNG format.
The sizes of these images are as follows:
lock backround: 320x480
wallpaper: 320x480 (just place this image in the theme folder. Ex: My Theme > Wallpaper)
dock: 320x67
icons: 60x60 (in the theme folder create a folder called icons for custom icons. Example: My Theme > Icons)
status bar: 320x20
Now with your image editing software, open up the images you wish to use in your theme and resize them to the correct sizes listed above.
Now once you are satisfied with your theme, open up your SSH software, in my case WinSCP and SSH into your iPod.
Now it's just a matter of drag and dropping your theme into your iPod.
Since I use winterboard this is where I will find my themes:
/private/var/stash/themes
this may be different to yours but you get the point. Then voila you have successfully created your first basic iPod theme. Just enable it in winterboard or whatever and there you go.
Hope I helped you out