code: private function readStyle()
      {
          $length = self::getUInt2d($this->data, $this->pos + 2);
          $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);
  
          // move stream pointer to next record
          $this->pos += 4 + $length;
  
          if (!$this->readDataOnly) {
              // offset: 0; size: 2; index to XF record and flag for built-in style
              $ixfe = self::getUInt2d($recordData, 0);
  
              // bit: 11-0; mask 0x0FFF; index to XF record
              $xfIndex = (0x0FFF & $ixfe) >> 0;
  
              // bit: 15; mask 0x8000; 0 = user-defined style, 1 = built-in style
              $isBuiltIn = (bool) ((0x8000 & $ixfe) >> 15);
  
              if ($isBuiltIn) {
                  // offset: 2; size: 1; identifier for built-in style
                  $builtInId = ord($recordData[2]);
  
                  switch ($builtInId) {
                      case 0x00:
                          // currently, we are not using this for anything
                          break;
                      default:
                          break;
                  }
              }
              // user-defined; not supported by PhpSpreadsheet
          }
      }
docs: Read STYLE record.