This option is for the Start menu, taskbar, and action center. You can also show an accent color for window title bars (the horizontal bar at the top of a window) and borders. You can choose an accent color, or have Windows select one (the accent color will provide a contrast with, or match, your wallpaper and Windows color).
The Custom mode gives you options between Light and Dark.
Note: The Light mode doesn't personalize the color of the Start menu, taskbar, and action center (that option is only available for Dark and Custom modes).
