In general, the appearance of an active or selected gadget changes to
inform the user the gadget state has changed.  A highlighting method is
specified by setting one of the highlighting flags in the gadget
structure's flags field.

Intuition supports three methods of activation or selection highlighting:

  * Highlighting by color complementing (gflg_gadghcomp)

  * Highlighting by drawing a box (gflg_gadghbox)

  * Highlighting by an alternate image or border (gflg_gadghimage)

  * No highlighting (gflg_gadghnone)

One of the highlighting types or gflg_gadghnone must be specified for each
gadget.

 highlighting by color complementing 
 highlighting by drawing a box 
 highlighting with an alternate image or alternate border