How to add a class on custom pages to {{body_class}}?

Classes like “.home-template” are added to the body tag, however I have custom pages like for example “art.hbs”, on this page the body tag has no class.

How can I add class “.art-template” to the body tag of art.hbs?
I have a few custom pages, so this would come in quite helpful.

Ref: Ghost Handlebars Theme Helpers: body_class