Galley has six colours, and its design system fits on one page because of it. Every colour has one job. One is the background, one is a raised surface, one is for text, one is for the action you are meant to take, one is for good news and one is for trouble. If a screen wants a seventh colour, the screen is wrong.
In Flutter that becomes a theme with named roles and nothing else. Widgets ask for the surface or the accent and never for a hex value. The part worth stealing is a small test that fails when a widget file contains a colour literal. The rule is cheap to enforce on six colours and hopeless on sixty.
Food apps are crowded with photographs, so the interface has to stay quiet around them. Six colours, one accent and generous space let a dish sheet, a menu tab and a courier map look like one product, even when the screens were built weeks apart.