TamperMonkey

// ==UserScript==
// @name         Markdown Justify
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://git.dbogatov.org/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    var css = '.md p { text-align: justify !important; } .md details { text-align: justify !important; }';
    var head = document.head || document.getElementsByTagName('head')[0];
    var style = document.createElement('style');

    head.appendChild(style);

    style.type = 'text/css';

    if (style.styleSheet){
        // This is required for IE8 and below.
        style.styleSheet.cssText = css;
    } else {
        style.appendChild(document.createTextNode(css));
    }

})();