There’s probably good reason for it.
I chalked it down to: web app dependency vulnerabilities, better keyboard support that isn’t clear to the eye, the tool being old and not being supported, the tool being updated by the vendor.
I could swear I saw in another URL that the old colour selections were there; Dracula + Shell were great.
Sure, there could be a good reason for the removal, except as is so often the case, I've seen nothing anywhere to indicate this. It seems this change first came about in a 10.36.x version of Jamf Pro, and I looked through all the Release Notes for any 10.36.x version and see nothing about this mentioned. For good measure I also looked through the 10.37 releases (all of them) and see nothing about this mentioned. It is possible I'm just not seeing it, but I don't think so.
If anyone can find anything that discusses this, I'd be happy to know. Until then, this appears to be another change from Jamf that isn't documented or explained, and seemingly has no reason to be.
The old themes are still available in the script panel of the extension attributes editor. I assume they forgot to update them there. Looking at them side-by-side I definitely prefer the old color themes. No idea why they removed them to begin with though, would be interested to know the official reason for the change.