The background changes color based on if you are giving a signature, like when you log into a site with your wallet. Or when you are dealing with a smart contract.
Security. Basically some scams depend on people just rushing through things and signing a smart contract when they thought they were giving a signature.
This helps make the system more user friendly to the average person and helps brings their attention to the system when a smart contract wants some or all of their funds. This hopefully limiting scams.