github-checklist-highlighter.user.js

// ==UserScript==
// @name     GitHub checklist highlighter 595491
// @version  5
// @grant    none
// @include  https://github.com/*
// ==/UserScript==

(function f() {
  var taskListItems = document.getElementsByClassName("task-list-item enabled");

  for (var i = 0; i < taskListItems.length; i++) {

    if (taskListItems[i].getElementsByClassName("task-list-item-checkbox")[0].checked) {
      taskListItems[i].style.backgroundColor = "rgba(0,255,0,0.15)";
    }
    else {
      taskListItems[i].style.backgroundColor = "rgba(255,0,0,0.15)";
    }
  }

  var formCheckboxes = document.getElementsByClassName("form-checkbox mt-0");

  for (var i = 0; i < formCheckboxes.length; i++) {

    if (formCheckboxes[i].querySelectorAll('[type="checkbox"]')[0].checked) {
      formCheckboxes[i].style.backgroundColor = "rgba(0,255,0,0.15)";
    }
    else {
      formCheckboxes[i].style.backgroundColor = "rgba(255,0,0,0.15)";
    }
  }
  
  setTimeout(function() { f(); }, 1e3);
})();