How to reply to a "you looked nice today" text message? Please help!?

Okay so for the past week or so I've been texting/messaging a boy from my village. I know him through mutual friends but don't properly know him, so all our texts have just been about what we're doing & bla bla bla.

Then after I see him today (& quickly say hi - didn't stop 'cause I was on my way to work) he texted me "You looked nice today :)" & now I don't know what to say back.

I'm notoriously bad with guys especially over text, plus this one is two years younger than me (I'm 21, he's 19) so I just don't have a clue! Please help!
How to reply to a "you looked nice today" text message? Please help!?
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