protected void paintIndeterminateState( Graphics g ) {
		g.setColor( Color.magenta );
		g.drawRect( 0, 0, getWidth() - 1, getHeight() - 1 );
	}
Paints the indeterminate state if the current LaF does not support displaying
the indeterminate state.
The default implementation draws a magenta rectangle around the component.
flatlaf-extras/src/main/java/com/formdev/flatlaf/extras/components/FlatTriStateCheckBox.java