In WordPress Dashboard I selected: Appearance -> Editor -> Styles -> Typography -> Manage Fonts -> and here I installed various available fonts. They are still there, but I had to reactivate them.
I don’t run too many plugins atm, and nothing updated at the time the problem occurred.
It happened while I was working on a post using the editor. Seemingly for no reason the whole website did a reset back to the default fonts and color palette. I didn’t touch any core files.
