Use the .align-content-*-center classes to align gathered items in the center on different screens.
.align-content-center:
.align-content-center
.align-content-sm-center:
.align-content-sm-center
.align-content-md-center:
.align-content-md-center
.align-content-lg-center:
.align-content-lg-center
.align-content-xl-center:
.align-content-xl-center