CSSRule.cssText

The cssText property of the CSSRule interface returns the actual text of a CSSStyleSheet style-rule.

Be aware that this property can no longer be set directly, as it is now specified to be functionally modify-only, and silently so. In other words, attempting to set it does absolutely nothing, and doesn't even omit a warning or error. Furthermore, it has no settable sub-properties. Therefore, to modify it, use the stylesheet's cssRules[index] properties .selectorText and .style (or its sub-properties). See Using dynamic styling information for details.

string = cssRule.cssText

body {
  background-color: darkblue;
}

let stylesheet = document.styleSheets[0];
console.log(stylesheet.cssRules[0].cssText); // body { background-color: darkblue; }