Some websites are really cursed in how hard they make overriding their styles.

Some things I usually do to handle these are using Stylus with Instant inject mode and some !important rules for a global dark theme.

If that also fails, I have an anti-Flash-of-Unstyled-Content userscript ready to toggle per-website as needed:

;(function() {

  const css = `
    html, body {
      background: #222 !important;
      min-height: 100vh !important;
    }

    body > * {
      visibility: hidden !important;
    }
  `

  const style = document.createElement('style')
  style.id = 'antiFOUC-block'
  style.textContent = css

  const container = document.head || document.documentElement
  container.prepend(style)

  window.addEventListener('DOMContentLoaded', () => {
    const block = document.getElementById('antiFOUC-block')
    if (block) block.remove()

    document.body.childNodes.forEach(n => {
      if (n.nodeType === 1) {
        n.style.visibility = ''
      }
    })
  })
})()

It works by preventing display of content before it's fully loaded. This won't make websites dark on its own, just prevent the flash.

(I have tested this with Violentmonkey on boardgamegeek.com plus a Stylus dark theme and it did solve it. With Dark Reader though it will still flash)

If even that fails I have a second one that will strip all inline and !important styles, but it rarely ever comes to that.

I hope this is helpful because being flashed with a fully white page makes me want to cry and punch my computer.