Board outline
Toggled from the Board menu, per board.
Outline on makes board edges explicit, which helps while you are arranging a layout or when boards sit close together.
Outline off is the quieter mode most people settle on. The board becomes invisible furniture: you see grouped icons on your wallpaper, and the Board widget fades in only when your pointer enters the board. This is the state the product is designed around — the tool disappears and the desktop remains.
Rounded corners are a separate toggle in the same menu.