Qt lists Light as a choice under font style for the Ubuntu font (along with light, regular, medium, bold, italic, italic bold) rather than as a separate font. So it doesn't let you do "bold of Ubuntu Light".
Qt lists Light as a choice under font style for the Ubuntu font (along with light, regular, medium, bold, italic, italic bold) rather than as a separate font. So it doesn't let you do "bold of Ubuntu Light".