A ViolentMonkey/TamperMonkey/GreaseMonkey script for keeping add/updating a meta theme-color tag for Outlook.com

outlook-theme-color.js

// ==UserScript==
// @name Set Outlook Theme Color
// @namespace Violentmonkey Scripts
// @match https://outlook.live.com/mail/*
// @grant none
// ==/UserScript==

function updateThemeColor() {
  let themeMeta = document.querySelector("meta[name=theme-color]");
  
  if (!themeMeta) {
    themeMeta = document.createElement('meta');
    themeMeta.innerHTML = `<meta name="theme-color">`;
    document.head.appendChild(themeMeta);
  }
  
  const themeColor = getComputedStyle(document.body).getPropertyValue('--headerBackground')
  themeMeta.setAttribute("content", themeColor);
};

const poll = (func, regularity) => {
  func();
  setTimeout(() => {
    poll(func, regularity);
  }, regularity)
};

poll(updateThemeColor, 1000);