So yes, you can change the variables of the shaders using the method included in the Ren'Py official documentation. I need to take the time to write up a cheat sheet for the shaders so people can see the variables without needing to dive into rows of code.
https://www.renpy.org/doc/html/textshaders.html#specifying-text-shaders
Most of everything is customizable, let me know if there's anything you can't easily change and I'll see if I can make an update to expose it or make some more easily utilized knobs.