Styles.css not applied after migration from Vaadin 14 to Vaadin 24

Question

I'm still working on the migration of our app from Vaadin 14 to Vaadin 24 and I've managed to have it more or less working. One of the last issues is that most of the CSS is not present.

I've followed these instructions to update my frontend directory from

frontend
└── styles
    ├── components
        └── [multiple CSS files]
    ├── lumo
        └── [multiple CSS files]
    ├── misc
        └── [multiple CSS files]
    ├── views
        └── [multiple CSS files]
    └── styles.css

to

frontend
└── themes
    └── my-app
        ├── components
            └── [only the files that where used in a CssImport with a "themeFor"]
        ├── lumo
            └── [no changes]
        ├── misc
            └── [no changes]
        ├── views<br>
            └── [no changes]
        ├── styles.css
        └── [moved here the files from the "components directory"]

I've removed every @CssImport from my app and added @Theme("my-app") to the class implementing AppShellConfigurator. I've also added an @import in the styles.css for every file not in the "components" directory.

The issue is that when I access my application there's barely any CSS. If I check in the style editor of my browser I get this:

Instead of this (I took this one from the app that's in production):

As you can see there's a few styles missing. What am I doing wrong?