Hugo module missing when building the ospo.zone website

When trying to build the ospo.zone website on my host, I get the following error:

Error: module "eclipsefdn-hugo-solstice-theme" not found; either add it as a Hugo Module or store it 
in "/home/boris/Projects/ecl_ospozone/node_modules".: module does not exist

It should probably be documented somewhere, or maybe it is documented somewhere.. Any hint?