Rewrite and use as you like.
// ==UserScript==
// @name Color Themes for 5ch
// @namespace http://tampermonkey.net/
// @version 1.2
// @author SenY
// @description Rewrite and use as you like.
// @match https://*.5ch.net/*
// @icon https://www.google.com/s2/favicons?sz=64&domain=5ch.net
// @grant none
// @license MIT
// ==/UserScript==
(function() {
'use strict';
/* default */
document.querySelector(":root").style.setProperty('--logo-navy', "#273f74");
//document.querySelector(":root").style.setProperty('--bg-lightgray', "#f3f3f3");
//document.querySelector(":root").style.setProperty('--text-black', "#000");
document.querySelector(":root").style.setProperty('--link-blue', '#1d69bf');
document.querySelector(":root").style.setProperty('--title-red', '#f23030');
document.querySelector(":root").style.setProperty('--meta-green', '#248041');
document.querySelector(":root").style.setProperty('--meta-new-orange', '#e59100');
document.querySelector(":root").style.setProperty('--line-gray', '#d9d9d9');
//document.querySelector(":root").style.setProperty('--pop-current-bg-gray', '#f6fdf5');
//document.querySelector(":root").style.setProperty('--reload-progress-gray', '#e0e0e0');
document.querySelector(":root").style.setProperty('--threadtitle-fsize', '23px');
document.querySelector(":root").style.setProperty('--meta-fsize', '12px');
document.querySelector(":root").style.setProperty('--comment-fsize', '17px');
/* custom */
document.querySelector(":root").style.setProperty('--text-black', "#fff");
document.querySelector(":root").style.setProperty('--bg-lightgray', "#333");
let s = document.createElement("style");
let t = '.post_hover { \
background : #222 !important; \
}';
s.innerHTML = t;
document.querySelector("head").appendChild(s);
// Your code here...
})();