After reading the hint Change the Dock's background and separator, I had been playing around with changing the .png files in /System » Library » CoreServices » Dock » Resources and had accidentally overwritten the backup copies of the png files I had copied to my desktop.Time Machine to the rescue! But I didn't want to restore the png files over the custom files I had in the folder. Also, Time Machine hadn't backed up since I overwrote the files on the desktop, so with the window open, I launched Time Machine and navigated back through time to when the original files existed, selected them, and hit Command-C to copy. Next I cancelled out of Time Machine, clicked on my desktop, and hit Command-V to paste. Voila! The original files were restored to my desktop ...

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