Contrast cards
Dark container bg-color-b — cards must stay dark-on-white
Boxed card inside bg-color-b. Text must stay dark on white.
Highlighted card inside bg-color-b. Same light surface.
Boxed + bg-color-c keeps its own C surface.
Dark container bg-color-a — boxed card
Boxed card inside bg-color-a. Dark text on the white card.