Loads a custom skin to Jstris, a userscript for Tampermonkey.

Jstris-loadCustomSkin.user.js

// ==UserScript==
// @name         Jstris Custom Skin Loader
// @namespace    http://tampermonkey.net/
// @downloadURL  https://gist.githubusercontent.com/Dinir/023cebc82d20cc06d0a790acf374d337/raw/3c7377b2948201a41090deddf1d6972bc131acba/Jstris-loadCustomSkin.user.js
// @version      1.2
// @match        https://*.jstris.jezevec10.com/*
// @grant        none
// @run-at       document-end
// @description  Loads a custom skin to Jstris.
// @author       Dinir Nertan
// @homepage     https://dinir.works
// @icon         https://i.imgur.com/FKXEPIn.png
// ==/UserScript==

// Run this script at 'document-end'. You can set it in 'Settings' tab next to 'Editor'.

(function () {
  'use strict'

  // store other skins in this manner: `name: ['address', size],`.
  const favSkins = {
    mf: ['https://i.imgur.com/t29rjjK.png', 24],
    bs48: ['https://i.imgur.com/8Fo3afG.png', 32],
  }

  // Define paths for your custom skin here.
  const customSkinPath = {
    bgImage: null,
    bgColor: null,
    bgColorHeight: '1000px',
    // type either `favSkins.name` or `['address', size]`.
    blockSkinAndSize: favSkins.bs48,
    ghostBlockSkinAndSize: [null, null]
  }

  const isArrayAndHasItem = instance =>
    Array.isArray(instance) && instance.some(v => v !== null)

  const loadCustomSkin = () => {
    if (customSkinPath.bgImage) {
      document.body.style.backgroundImage = `url(${customSkinPath.bgImage})`
      document.body.style.backgroundSize = '100%'
    }
    if (customSkinPath.bgColor) {
      document.getElementById("app").style.backgroundColor = customSkinPath.bgColor
      document.getElementById("app").style.height = customSkinPath.bgColorHeight
    }
    // if (typeof loadSkin !== 'undefined') {
      if (isArrayAndHasItem(customSkinPath.blockSkinAndSize)) {
        loadSkin(customSkinPath.blockSkinAndSize[0], customSkinPath.blockSkinAndSize[1])
      }
      if (isArrayAndHasItem(customSkinPath.ghostBlockSkinAndSize)) {
        loadGhostSkin(
          customSkinPath.ghostBlockSkinAndSize[0],
          customSkinPath.ghostBlockSkinAndSize[1] ?
          customSkinPath.ghostBlockSkinAndSize[1] :
          customSkinPath.blockSkinAndSize[1]
        )
      }
    // }
  }

  window.addEventListener('load', loadCustomSkin)
})()