How to include shopware stylesheets within a main or custom module of an app?

we are developing an app that extends shopware admin via a main module, defined in manifest.xml.

So we load an Iframe with a couple of forms, it works, functionality is fine, but the styles are external.
In the Shopware documentation on modules it says that one can use the shopware styles :

It is not quite clear how exactly to do it, e.g. the file structure is now different in shopware/administration repository. Is there any shopware instruction / best practice / experiences that I am not aware of? Would be grateful for some hints.

You could use the Meteor component library that includes components that resemble the original components in the administration including the styling.