Sorry, I don't think I'll be able to help, since I could not get it to compile multiple CSS files (using mini-css-extract-plugin) with my setup in a satisfying way. The problem is that my CSS files are imported from the JS files, so the _layers_ solution only works if I multiply the main JS entrypoint, but then obviously it also compiles the whole JS again and again for every theme.

I worked around it by adding an entry point for every theme, each listing every single CSS file and defining `layer: theme`, so I could define different loader rules based on the `issuerLayer`.