Imagine Adrien noticing how often Theo seems to be around and how he’s always doing a different job and, because he hasn’t helped the poor guy out with the whole Copycat fiasco, he takes pity on him. He looks around online and in the general area of Theo’s warehouse and finds out that Theo does sell his art, mainly intricate little sculptures, but also larger statues and even some paintings. So every month Adrien buys a couple (often with very slight changes to his appearance so it isn’t completely obvious who he is) and then gives them to his classmates.
Alya’s stoked about it because she recognises the craftsmanship as the same guy who made the Ladybug/Chat Noir statue in the park (also because Theo’s name is on everything…). Marinette’s also really impressed because 1) Adrien’s giving her stuff and 2) she really does think Theo is an amazing artist. So Adrien ends up giving a lot of the things he buys to Alya and Marinette because they seem to appreciate them the most. Alya’s favourite is unsurprisingly a mini replica of the Ladybug from the statue, but Marinette turned down Adrien’s offer to have one as well.
And in the end, Theo is confused but grateful to the mysterious buyer who comes by once a month and always pays slightly more than he’s actually asking for because he can finally quit one of his extra jobs.