Please go to My Site → Customize → CSS (or Additional CSS), then add all of the following code to the bottom of that window.
/* This CSS centers the items under "Shop By Category" on Storefront homepage.
* 10755457-hc (jw)
*/
.storefront-product-categories .products.columns-3 {
display: flex;
justify-content: center;
}
Tip: keep the /*
comment */
in the code to later know what this code does.