When viewers were first introduced to the young Stark daughter, she was being groomed — much to her chagrin — to become a "proper" lady. However, the few times she was actually shown in courtly attire, her hair and gowns were visibly disheveled.
Following her father Ned's beheading, Arya's hair was chopped off by her father's friend Yoren — who also gave her the idea of keeping a kill list — and she began dressing in "boyish" clothing in order to covertly travel through Westeros.
She maintained this style up until starting her training with the Faceless Men, at which point she typically appeared in either the uniform of the House of Black and White or traditional Braavosi garb.
Now that she has reclaimed her Stark identity and is (possibly) journeying back to Westeros, it looks as though her clothing will mirror this return to her roots — sneak peaks of season seven have shown her decked out in Northern attire.
See the full gallery of Arya's costumes above.