focusColor property
final
The color for the checkbox's Material when it has the input focus.
If overlayColor returns a non-null color in the MaterialState.focused state, it will be used instead.
If null, then the value of CheckboxThemeData.overlayColor is used in the focused state. If that is also null, then the value of ThemeData.focusColor is used.
Implementation
final Color? focusColor;