party.js

function party() {
  const colors = Array.from(document.styleSheets).find(stylesheet =>
    stylesheet.href.endsWith("colors.css")
  ).cssRules[0].style;
  const colorVariables = Object.values(colors);
  const sourceStyles = document.documentElement.style.getPropertyValue(
    colorVariables[0]
  )
    ? document.documentElement.style
    : colors;
  const nextColor = colorVariables.map((color, index) => {
    const nextIndex = (index + 1) % colorVariables.length;
    return [color, sourceStyles.getPropertyValue(colorVariables[nextIndex])];
  });
  for (const [property, color] of nextColor) {
    document.documentElement.style.setProperty(property, color, "important");
  }
  const colorBrowsers = Array.from(document.querySelectorAll("browser")).filter(
    browser =>
      browser.src &&
      Array.from(browser.contentDocument.styleSheets).some(stylesheet =>
        stylesheet.href.endsWith("colors.css")
      )
  );
  for (const browser of colorBrowsers) {
    for (const [property, color] of nextColor) {
      browser.contentDocument.documentElement.style.setProperty(
        property,
        color,
        "important"
      );
    }
  }
}

setInterval(party, 100);