src/language-css/printer-postcss.js/isDetachedRulesetDeclaration
function isDetachedRulesetDeclaration(node) {
  // If a Less file ends up being parsed with the SCSS parser, Less
  // variable declarations will be parsed as atrules with names ending
  // with a colon, so keep the original case then.
  return (
    node.selector &&
    node.selector.type !== "selector-root-invalid" &&
    ((typeof node.selector === "string" && /^@.+:.*$/.test(node.selector)) ||
      (node.selector.value && /^@.+:.*$/.test(node.selector.value)))
  );
}