1. National law
2. Facebook TOS
3. Group rules
But the legislative power, to to speak, at the group level is quite weak. They can further restrict according to some values, which is fine as it is. Freedom of association. They can't control the UI.
"In iOS, iPadOS, and macOS, icons are square, and the system applies masking to produce rounded corners that precisely match the curvature of other rounded interface elements throughout the system and the bezel of the physical device itself."
Source: Apple https://developer.apple.com/design/human-interface-guideline...
lol