It's pretty hard to go wrong in this situation so long as you're not flippant. Gratefulness is normally the best way to respond to gratefulness, so maybe thank them and say you do it to share them and you're always glad to be appreciated. Adjust the strength of this sentiment based upon the degree of its truthfulness. Chance is you're probably happy to leave it at that, but if you're concerned and want to engage them further make sure they're aware of this otherwise it'll just end there.