Hello,

Please add this code to the Theme Settings > Custom CSS > Global:

.product-grid-item .price del {
	text-decoration: none;
}

.product-grid-item .price del > span {
	text-decoration: line-through;
}

.product-grid-item .price del:before,
.product-grid-item .price ins:before {
	margin-right: 5px;
}

.product-grid-item .price del:before {
	content: "WAS";
}

.product-grid-item .price ins:before {
	content: "NOW";
}

Best Regards