i think having a dbCLS in the loop after dbSync would help
anyhow, if you dont want to use dbCLS, you can use dbMakeObjectCube ( ... ) (that's it's name? whatever) then dbHideObject ( ) to hide it, then SetBackdropColor ( ) to set the color of the background, then, continue working normally without any need for dbCLS command, because having a 3d object in the scene (even though it wont be visible) will make GDK clear the screen every time before rendering, instead of explicit clearing on dbCLS so something like this should work
void DarkGDK ( void )
{
dbSyncOn ( );
..
..
dbMakeObjectCube ( 1, 5 ); //create a dummy cube
dbHideObject ( 1 ); //hide the dummy cube
dbSetBackdropColor ( dbRGB ( r, g, b ) ); //rgb color of the background
//ok now rendering is fine
while ( LoopGDK ( ) )
{
//whatever your code is..
//..
//..
dbSync ( );
}
}