Body and Hero classes settings as default in a new blog item


I am new to Grav but I think it is a great product.
I am creating my own blog with the Quark theme.
I want to adjust the settings of Body and Hero classes by default so that I don’t have to set them manually every time. In which file (twig?) Should I do this and what should I set.

It’s about :
hero_classes: text-light title-h1h2 parallax overlay-dark-gradient hero-large.
body_classes: “header-dark header-transparent”
Thanks for helping me out.