haddock-project --css does not properly apply the stylesheet in multi-package projects

[ozkutuk]

Describe the bug
cabal haddock-project --css does not properly apply the provided stylesheet in multi-package projects. There are two problems that I can see:

1. The custom stylesheet is not applied to the index page at all
2. The style of the package haddocks are broken when the packages reside in their individiual directories (and not at the root of the project)

I suspect the issue is not specific to haddock-project and probably also applies to haddock command too.

To Reproduce
I have created a small reproducer here. Follow the repo's README for details.

Expected behavior
Both the index page and each of the package haddocks have the provided stylesheet applied.

System information

• Operating system: NixOS
• cabal: head
• ghc: 9.6.6