CSS box at bottom of settings. You should do something like this:

body, button, input {
	font-family: "examplefont";
}
@font-face {
	font-family: "examplefont";
	src: url(https://example.com/examplefont.ttf);
}

Be warned, some domains will be blocked and thus the font won't render! (I speak from my own experience, had to get a lot of help because of that domain block thingy...)

using css! its at the very bottom of the theming part of settings

uhh i think they use sites like instafonts ig? not sure