The width of the border surrounding each toggle button.
This applies to both the greater surrounding border, as well as the borders rendered between toggle buttons.
To render a hairline border (one physical pixel), set borderWidth to 0.0. See BorderSide.width for more details on hairline borders.
To omit the border entirely, set renderBorder to false.
If this property is null, then ToggleButtonTheme.of(context).borderWidth is used. If ToggleButtonsThemeData.borderWidth is also null, then a width of 1.0 is used.
final double borderWidth