woocommerce/plugins/woocommerce-admin/client/dashboard/store-performance/style.scss

.woocommerce-dashboard__store-performance {
	margin-bottom: $gap-large;

	@include breakpoint( '<782px' ) {
		border-width: 0;
	}

	.woocommerce-summary {
		background-color: $gray-100;
		margin: 0;

		@include breakpoint( '<782px' ) {
			&.is-placeholder {
				border-top: 0;
			}

			&:not(.is-placeholder) {
				.woocommerce-summary__item-container:first-child {
					.woocommerce-summary__item {
						border-top: 1px solid $gray-400;
					}
				}
			}
		}
	}
}