Being a new user I imported a project that was in European Typewriter, a font that Scrivener doesn’t have the ability to render italics or bold.
With help from this forum that was sorted out but I opened a new project [cmd-n] to import something else I’m working on and the font on the Untitled document that I got using a blank template was in European Typewriter. I have been through Scrivener>Preferences and Project>Text preferences to no avail.
The default font for new text documents is configured in Scrivener/ Preferences/ Formatting.
This is a global Scrivener setting (not per-project), so it will apply to all your newly created text documents on all projects.
If your project has already any existing documents you want to normalize to your personal settings, they can be converted to the default using the Documents/ Convert Formatting to Default Text Style command.