Flexbox center columns

ul {
	@include media(">=tablet") {
		display: flex;
		flex-wrap: wrap;
		justify-content: center;
		margin-left: -16px;
		margin-right: -16px;

		li {
			margin-left: 16px;
			margin-right: 16px;
			flex-basis: calc( 50% - 32px );

			@include media(">=medium") {
				flex-basis: calc( 25% - 32px );
			}
		}
	}
}