code: private function writeStyle()
      {
          $record = 0x0293; // Record identifier
          $length = 0x0004; // Bytes to follow
  
          $ixfe = 0x8000; // Index to cell style XF
          $BuiltIn = 0x00; // Built-in style
          $iLevel = 0xff; // Outline style level
  
          $header = pack('vv', $record, $length);
          $data = pack('vCC', $ixfe, $BuiltIn, $iLevel);
          $this->append($header . $data);
      }
docs: Write Excel BIFF STYLE records.