js

const styleSheet = [];
const styleSheets = document.styleSheets;

for (let i = 0; i < styleSheets.length; i++) {
  styleSheet.push(styleSheets[i]);
}

js

const allCSS = [...document.styleSheets]
  .map((styleSheet) => {
    try {
      return [...styleSheet.cssRules].map((rule) => rule.cssText).join("");
    } catch (e) {
      console.log(
        "Access to stylesheet %s is denied. Ignoring…",
        styleSheet.href,
      );
    }
  })
  .filter(Boolean)
  .join("\n");