This is actually really simple, although there's one tricky part that stumped me for a couple of hours. I couldn't find one place anywhere on the internet that explained how to fix it, so here it goes. Also, please don't tell me how much you hate Pogo Plank, that has nothing to do with this thread.
Requirements: PogoPlank, Photoshop, the ability to SSH.
1. SSH into your iPod/iPhone. Navigate to /Library/PogoPlank/Themes.
2. Copy the 'Default' theme onto your computer.
3. Create a new folder and name it whatever you want your theme to be called. Copy all the files from the Default theme into this new folder.
4. Within the new folder will be 5 files: folder.png, list.png, Theme.plist, Thumbs.db (ignore this), and wheel.png. Obviously the *.png files correspond to the folder, list, and wheel icons. Open each of these up in Photoshop (I don't know if you can do it with other programs, maybe try The Gimp) and edit them how you want to create your theme. Make sure to preserve their size and to save them as a *.png.
5. Once you are finished editing the icons, open up the Theme.plist file in notepad or any other word processing program. Look for a line that goes like this:
Rename the part that says 'Default' to match exactly the name of your theme (whatever you named the theme folder). This is very important to do, otherwise you won't be able to activate your theme.
6. Now SSH your new theme folder into /Library/PogoPlank/Themes. I recommend keeping the Default theme on your computer so you can make more themes in the future.
7. Open up PP Settings on your iPod/iPhone, go to the themes section, and activate your theme. Everything should work fine.
That's pretty much it. I hope it helped someone. Anyway, if anybody needs me to, I"ll put up some pictures to make everything clearer. Good day.