That led me to https://www.folklore.org/Desk_Ornaments.html which is a very fun read. Interesting to note that the UI style of the DAs is actually not consistent at all, some have round corners and some don't.

I particularly like this Bill Atkinson tidbit at the end:

Bill Atkinson complained to me that it was a mistake to allow users to specify their own desktop patterns, because it was harder to make a nice one than it looked, and led directly to ugly desktops. [...] So he made MacPaint allocate a window that was the size of the screen when it started up, and filled it with the standard 50% gray pattern, making his own desktop covering up the real one, thus protecting the poor users from their rash esthetic blunders, at least within the friendly confines of MacPaint.

(He was totally right, making your own desktop patterns was fun but the standard checkerbard was far and away the best choice.)