woocommerce/plugins/woocommerce-admin/client/marketing/coupons/card/style.scss

.woocommerce-admin-marketing-card {
	.components-card__header {
		.woocommerce-admin-marketing-card-subtitle {
			font-weight: 400;
			margin-top: 4px;
			color: $gray-700;
		}
	}
}