// flex 垂直水平居中
@flex:{
  display: flex;
}
@flex-center-row :{
  display: flex;
  align-items: center;
  flex-direction: row;
}
@flex-center-col :{
  display: flex;
  align-items: center;
  flex-direction: column;
}
@flex-center :{
  display: flex;
  justify-content: center;
  align-items: center;
}