Light theme for Kibana users that can't change it. It changes dark CSS files to use light alternatives, forces CSS with !important or changes style attributes if that's not possible.

userscript.js

// ==UserScript==
// @name           Elastic Light Theme
// @description    Replaces dark themes with light ones
// @author         Elias Dorigoni
// @include        https://your.kibana.url/*
// @version        1.0
// ==/UserScript==

window.clearTimeout = window.clearTimeout.bind(window);
window.clearInterval = window.clearInterval.bind(window);
window.setTimeout = window.setTimeout.bind(window);
window.setInterval = window.setInterval.bind(window);


function replaceDarkThemes() {
  Array.from(document.querySelectorAll('link[type="text/css"][href$="dark.css"]'))
    .map(darkTheme => {
      let head = document.getElementsByTagName('HEAD')[0]
      let link = document.createElement('link')

      link.rel = 'stylesheet'
      link.type = 'text/css'
      link.href = darkTheme.getAttribute('href').replace('dark', 'light')
      head.appendChild(link)
      darkTheme.remove()
    })
}

function extractElementDarkClasses(element) {
  return Array.from(element.classList).filter(className => className.includes('--dark'))
}

function replaceDarkClassNames() {
  Array.from(document.querySelectorAll('*')).map(el => {
    let darkClasses = extractElementDarkClasses(el)
    if (darkClasses.length == 0) {
      return
    }

    darkClasses.map(className => {
      let newClassName = className.replace('dark', 'light')
      document.querySelector('.' + className).classList.replace(className, newClassName)
    })
  })
}

function addInlineStyleBlock() {
    const element = document.createElement('style')
    element.textContent = "\
html {color: #555 !important;background-color:white !important}\
.dscFieldList--popular {background-color:#ccc !important}\
.dscFieldListHeader {background-color:#ccc !important}\
.globalFilterItem:not(.globalFilterItem-isDisabled){background-color:rgba(0,119,204,.2) !important}\
.globalFilterItem{color:inherit !important}\
.kbnFilterButtonGroup{background-color:rgba(0,119,204,.2) !important}\
.embPanel__title{color:inherit !important}\
.kbnTypeahead .kbnTypeahead__popover{background-color:#fafafa !important}\
.kbnTypeahead__item.active{background-color:rgba(0,119,204,.2) !important}\
.kbnSuggestionItem__text{color: #555 !important}\
.kbnTypeahead .kbnTypeahead__popover .kbnTypeahead__item.active .kbnSuggestionItem__text{color:#333}\
.css-fmamwg-euiAvatar-s-space{color:white !important}\
"
    document.head.append(element);
}

function replaceInlineStyles(selector, styleContent) {
  let el = document.querySelector(selector)
  if (el != null) {
    el.setAttribute('style', styleContent)
  }
}

replaceDarkThemes()
replaceDarkClassNames()
addInlineStyleBlock()

window.setInterval(function() {
  replaceInlineStyles('.echChartBackground', 'background-color:white')
}, 1000)