D.N.Angel, Vol. 1 by Yukiru Sugisaki

Daisuke Niwa appears to be your average high school student, but there's one secret that he and his family have that makes him anything but normal. On his fourteenth birthday, his secret genetic "condition" that even he didn't know about makes him transform into Dark Mousy - a famous Phantom Thief that has been notoriously stealing works of art for centuries, whenever he thinks about his crush Risa Harada. Now Daisuke must carry on the family legacy of becoming Dark and steal numerous art pieces while balancing school and without getting caught by the police or Satoshi Hiwatari - one of Daisuke's mysterious classmates!