small spoiler : it is on purpose, trust !!
NOVEL SPOILER
The reason is actually sad : he wears plain stuff to blend in, to not stand out. Because he had to stop pursuing art, he is kind of putting restraints on himself, hence why he is wearing striped shirt. They are not too basic like a plain tshirt, but the design is not too outstanding either. In a way, the stripes reminds me of a prison he is locking himself into

The way the MC is represented is quite disappointing tbh, everybody has this unique design and style, and here he is with his bland everything, I find the two gothic/punk workers to be far more interesting than the main characters I'm sorry nonetheless the story is interesting and I'll keep reading ╮( ̄▽ ̄)╭