To change the background colour of a control after a window/dialog has been created.
HANDLE is the window handle of the control obtained by using %lc. COLOUR is the
RGB colour value (see RGB@).
Unless the colour is to be varied you can set the background colour of a control using
%`bg before the control when the window is created.
A call to SET_CONTROL_BACK_COLOUR@ can be made in the callback function for the
control in question.