.theme_tab_item {
  display: block;<--------change to inline-block
  float: left;<-----------remove
  //other codes
}

.theme_tabs{
    list-style: none;
    padding-left: 0;
    display: inline-block;
    max-width: 1200px;
    margin: 0 auto;
}

.theme_tabs{
  display: flex;
  justify-content: center;
}