body, html
{
  height: 100%;
}

.outer {
  display: flex;
  width: 100%;
  height: 100%;
  justify-content: center;
  align-items: center;
}

.middle {
  display: table-cell;
  vertical-align: middle;
}

.inner {
  margin-left: auto;
  margin-right: auto;
  width: 400px;
  /* Whatever width you want */
}