You need to use custom CSS. Please check https://discourse.joplinapp.org/t/introduction-to-customising-joplin-userchrome-css-userstyle-css for detailed instructions.
The following code in userchrome.css will enforce system sans-serif fonts everywhere in the application layout.
*,
:before,
:after {
font-family: sans-serif !important;
}
However, it will also break icons, so you need to either exclude those (e.g. with :not), or instead target specific elements and change the fonts only there.