If iconOn or iconOff are not null, they will be displayed in lieu of the respective textOn and textOff. The values for textOn and textOff are used as semantic labels for the icons for accessibility. The icons in the example are not in the default material icon set, but are from [Material Design Icons] (https://materialdesignicons.com/), available on pub.dev here.
contentSize drives the height of the text or icons.