In a css you need add @font-face rules, then define font-family for a certain tag e.g.
@font-face{
font-family: "DancingScript-Regular";
font-style: normal;
src: local(/system/fonts/DancingScript-Regular.ttf);
}
p{font-family: DancingScript-Regular;}
All it is well described there: https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face
But it works not for all fonts. So I propose add link for each font into the Preferences -> HTMLHeader and footer -> Header.
After just type desirable font-family in a tag property in your note or in Preferences -> CSS styles -> User style.