Customizable package (theming a flake-managed widget bar)

I just want to know from you how you (or how you would) pass options to packages you develop with flakes.

I am currently writing a widget bar using Astal and then import the flake in my NixOS flake and use the package.default output in home.packages. I'd like to also style this bar according to my Stylix theme, how do you do something like this?