Answers for "make div center inside div"

center a div in css

.container {
  display: flex;
  justify-content: center;
  align-items: center;
}

align a div in center of another div

#container {
    width: 640px; /*can be in percentage also.*/
    height: auto;
    margin: 0 auto;
    padding: 10px;
    position: relative;
}