To make center categories, please find the file https://sowingseed.org/assets/css/style.css And delete line number from 132 to 159.

And add below CSS CODE

.home-category-box {
    height: 50px;
    overflow: hidden;
    position: relative;
    margin-bottom: 20px;
    text-align: center;
.home-category-box img {
    height: 50%;
    width: 50%;
    position: relative;
    vertical-align: middle;
    display: none
.home-category-box .title > a {
    color: #fff;
    font-size: 16px;
    height: 100%;
    /* position: absolute; */
    text-align: center;
    top: 0;
    width: 50%;
    line-height: 50px;
    background-color: #67C22C;
    -webkit-transition: all 0.4s ease-in-out;
    transition: all 0.4s ease-in-out;
    display: inline-block;

Hope it will be work