This variable has meaning only on graphical systems such as Windows. Some graphical systems (such as Windows) use a "background brush" when they resize a window. By default, the background brush color for ACUCOBOL-GT is black ("0", off, false, no). If you have arranged your default background to be white, you will see a black flash when you resize the window. This does not affect the final appearance of the window, but is briefly noticeable while the window is being redrawn.
Set WHITE_FILL to "1" (on, true, yes) to cause ACUCOBOL-GT's background brush to be set to white instead of black. Doing this will also cause the initial screen that ACUCOBOL-GT paints to be white instead of black.