Serving base theme project resources using maven

Hi.

We currently have 2 Vaadin 7.1.5 Projects which are packaged as war and deployed on a tomcat 7 with mainly the same theme in the webapp/VAADIN/themes folder. Now we would like to extract a base theme project with a reusable scss and some images. The result should be equal to what reindeer provides: A reuasable theme. But contrary to VAADIN/themes/reindeer the images in VAADIN/themes/base-theme are not served. The final styles.css is somehow built correctly at compile time.

I’ve been using this tutorial to some extend but I don’t know how either the scss compiler finds the base-theme, nor how serving static content/images from the - here by maven - produced jar.
https://vaadin.com/wiki/-/wiki/Main/Creating%20a%20reusable%20Vaadin%20theme%20in%20Eclipse

Now, theres this base-theme project, that just contains the src/main/resources/VAADIN/themes/base-theme.scss including and extending reindeer. And also some images, that are referenced by the base-theme.scss by relative path. Packaging is jar. The deployment assembly maps /src/main/resources/VAADIN to VAADIN.
This base-theme is a dependency of the ‘entrypoint’ project, which contains the vaadin application archetype with the com.vaadin.ui.UI. This entrypoint project defines a theme “project-specific” itself by extending the base-theme whilst @including the base-theme mixin. This theme is used with @Theme(“project-specific”).

At runtime the images from VAADIN/themes/reindeer/… get served whereas VAADIN/themes/ yields:
INFO: Requested resource
[/VAADIN/themes/project-specific/img/someimage.png] not found from filesystem or through class loader. Add widgetset and/or theme JAR to your classpath or add files to WebContent/VAADIN folder.

Could someone please explain, why the vaadin-themes.jar and our base-theme.jar are handled differently, even if both contain the same folder structure?
Is this a vaadin, maven or tomcat thing?

Thanks a lot.