src/language-css/printer-postcss.js/maybeToLowerCase
function maybeToLowerCase(value) {
  return value.includes("$") ||
    value.includes("@") ||
    value.includes("#") ||
    value.startsWith("%") ||
    value.startsWith("--") ||
    (value.includes("(") && value.includes(")"))
    ? value
    : value.toLowerCase();
}