This was a request I got on stream on Monday: a shader extension!
This extension lets you use images or rectangles to define parts of the screen to shade.
Grab it at riknoll/arcade-shader
Here is an example for that gif above:
And here’s another example that @shakao made on how to use it to make shadows for sprites
warning: the more of the screen you shade, the slower your game will be! that GIF above runs at 15 FPS on my pygamer and it only has two sprites. To make your game faster, use shaders lightly and use rectangles where possible (they are much faster). Also, avoid double-shading the same area.