getAllCssRuleBySelector function
Returns a List<CssRule> for targetSelector.
Implementation
List<CssRule> getAllCssRuleBySelector(
    Pattern targetSelector, CssStyleSheet? sheet) {
  if (sheet == null) return [];
  if (targetSelector is String) {
    var s = targetSelector.trim().toLowerCase();
    if (s.isEmpty) return [];
    return _getAllCssRuleBySelectorString(s, sheet);
  } else if (targetSelector is RegExp) {
    return _getAllCssRuleBySelectorRegExp(targetSelector, sheet);
  } else {
    throw StateError('Invalid targetSelector: $targetSelector');
  }
}