Add the following snippet to: Appearance > Customize > Additional CSS to do so:
.blog-menu {
text-align: center;
}
.blog-menu .sub-menu {
text-align: left;
}
.blog-menu > li {
float: none;
display: inline-block;
}
.blog-menu a {
padding-left: 15px;
padding-right: 15px;
}