There are a couple of themes on androart.net and android-hilfe.de.

You can customize the framework-res.apk yourself. Copy it to your computer and open it with ZIP/RAR/7z and modify the graphics or XML files. After you're done just insert the modified files back into the archive. Don't rearchive them as you'll have to resign the package, instead just use the original .apk and drag-drop your modified files in there. Copy the modified .apk back to the phone, reboot and you'll see the changes.
The XML layout files require more work because they're in binary mode and you have to use some more programs to read them properly, edit and reencode them.

Good luck.

Oh and BTW, don't use files for the droid on your Milestone because you may brick it. Even if it's just a theme or .jar file it might brick your phone.