1. Select an ApplicationWindow item in Design mode.
2. Click on the arrows in the spin box for opacity.
   They will increase or decrease the value by 1.

Usually, values for opacity are in the range from 0 to 1, so it would be better to have smaller steps, e.g. 0.1.

On a side note: Does it make sense that the spin box allows values outside this range at all? They are syntactically correct, but do they make sense?