There is a list of pre-installed fonts in Special:ThemeDesigner. If you want to use one of those, it is as simple as changing the option in ThemeDesigner and saving the change; not CSS needed. If you want a different font, try using #mw-content-text for the selector.
Edit: I see in the image that you are currently using .WikiaArticle. FandomDesktop doesn't use that class so that is why the CSS as you currently have it doesn't work anymore.