For changing typography first you have to goAppearance -> Typography.
Click Add New button for adding your desired font-family, font-weight, font-size, color & CSS selectors like as h1, h2, …..h6, p, li, a, span, body etc. also, you are able to add classes & IDs.
After done above click save
N.B. You can’t not modify StartNext custom CSS for this.