emoji-one-color.user.js

// ==UserScript==
// @name           Emoji One Color Font
// @namespace      http://dango.space
// @description    Insert Emoji One Color font into css on all pages
// @match          *://*/*
// @require        https://ajax.googleapis.com/ajax/libs/jquery/2.2.2/jquery.min.js
// @grant          none
// ==/UserScript==

this.$ = this.jQuery = jQuery.noConflict(true);

$("body").css("font-family", function(index,currentValue){
  if (currentValue.match(/Emoji One Color/i)) {
    return;
  } else if (typeof currentValue == 'undefined') {
    return "Emoji One Color";
  } else {
    return '"Emoji One Color",' + currentValue;
  }
});

Where is this script supposed to be executed?

It would typically be run in a browser extension such as Greasemonkey/Tampermonkey, however I no longer use this personally.

What do you use now as a replacement/substitute?