Toggling dark mode manually via javascript 

Building upon my previous explorations, I wanted to let people without an OS-wide dark-mode toggle try it out themselves directly.

For that, we use the styleSheets.cssRules browser API.

Try it: toggle dark/light!

function toggleDark() {
  let stylesheets = document.styleSheets;
  for (let sheet of stylesheets) {
    try {
      ruleList = sheet.cssRules;
      if (ruleList == undefined) {
        continue;
      }
    } catch (e) {
      // Catch CORS denials and skip stylesheet
      continue;
    }
    let dark = "(prefers-color-scheme: dark)"
    let light = "(prefers-color-scheme: light)"
    for (let rule of ruleList) {
      /* type 4 == CSSMediaRule */
      if (rule.type !== 4) {
        continue;
      }
      let text = rule.media.mediaText;
      if (text.slice(0, 22) == "(prefers-color-scheme:") {
        rule.media.mediaText == dark ?
          rule.media.mediaText = light :
          rule.media.mediaText = dark
      }
    }
  }
}

Caveat: If testing this on local file:// URIs, CORS will bite you, either on Chrome or FF. On FF, at least you can remedy this by setting privacy.file_unique_origin=false in about:config. See CVE-2019-11730.