How to change ugly Big Sur colorful starting background (on multi-user machine)?

(Question is in the title)

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This machine has more than 1 user, so the starting background cannot be changed through editing the files on "/Library/Caches/Desktop Pictures", even when the images on both folders (one for each user) are exactly the same.

So, again, the question: How can that ugly colourful Big Sur background in the starting screen (where all users are shown) be changed?

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Notes:

1. On 2019 MBP
2. I have admin privileges
3. I do not mind having to mess with System files, as long as there is no irreparable damage (if I can reinstall the OS from a USB stick or whatever in case of an accident, I'm more than fine).

Thanks in advance!