PostProcessors can be used to apply a shader to the entire screen. It is recommended you use the ScreenShader to build your post processor shader.
The screen texture comes through as this uniform
uniform sampler2D u_image
Post processor shaders get some default uniforms passed to them
uniform float u_time_ms - total playback time in milliseconds
uniform float u_elapsed_ms - the elapsed time from the last frame in milliseconds
uniform vec2 u_resolution - the resolution of the canvas (in pixels)
Custom uniforms can be updated in the PostProcessor.onUpdate