If you want to fill the screen with white just add a layer with a non-transparent (white) background on top of everything, and adjust its opacity in events. To process a shader on the full screen, create a window-sized Canvas, enable 'grab layout before drawing', and add a shader to the canvas object.
By default Construct uses linear sampling, which is a way of smoothly rotating and stretching sprites, but its well known side effect is pixelly sprites at floating-point positions look slightly blurred. To restore pixelly movement, rotation and stretching, set sampling to Point in application properties.