I’ve taken into advice: https://gohugo.io/themes/creating/
If you’re creating a theme with plans to share it with the community,
use relative URLs since users of your theme may not publish from the root of their website
Does this apply to internal shortcodes as well (?).
I currently see no way how to use {{< figure … in a project if it is not guaranteed that the project will be (and stay) in the root.
Looks like recreating the figure shortcode in the template for the project the only way…