css

@media (min-width: 500px) {
  body {
    color: blue;
  }
}

js

let myRules = document.styleSheets[0].cssRules;
let text = myRules[0].conditionText;
console.log(text); // "(min-width: 500px)"
text = "(min-width: 400px)";
console.log(text); // "(min-width: 400px)"