I spent some time examining Stripe's login page decorative background and concluded it's built around two ideas: 1) a diagonal line that crosses through the midway point between the input boxes and the submit button, and 2) vertical lines spaced out half the width of the card. There are then slight variations around those two ideas that make the page more interesting, which I compare to how music is more interesting when the sound has variation around an aesthetically-pleasing tone/chord (e.g. timbre).