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busybox0 / Jenkins colorize header depending on instance 

// ==UserScript==
// @namespace     https://openuserjs.org/users/busybox0
// @name          Jenkins colorize header depending on instance
// @description   If you work with several jenkins servers, it's easy to get into a muddle, as only url shows what server you are on. Having top frame colored differently helps to quickly distinguish where you are.
// @copyright     2019, Max P. (https://openuserjs.org/users/busybox0)
// @license       GPL-2.0-or-later
// @version       1.0
// @match      https://*.some.example.com/*
// @grant none
// ==/UserScript==

// ==OpenUserJS==
// @author busybox0
// ==/OpenUserJS==

var u = document.location.origin;
var x = document.getElementById("breadcrumbs"); // here you can try "breadcrumbs" and "header" to decide what you like more. "header" is better with dark colors.

// This is variation if you prefer solid colors over gradiets. Put it below.
// x.style.backgroundColor = "lightgray";
switch (u) {
  case "https://jenkins-1.some.example.com":
    x.style.backgroundImage = "linear-gradient(to right, #f6faf2, #f6faf2, violet)";
    break;
  case "https://jenkins-2.some.example.com":
    x.style.backgroundImage = "linear-gradient(to right, #f6faf2, #f6faf2, yellow)";
    break;
  case "https://jenkins-3.some.example.com":
    x.style.backgroundImage = "linear-gradient(to right, #f6faf2, #f6faf2, #f6faf2, #f6faf2, Green, DarkBlue, #f6faf2)";
    break;
}