。
A black party dress suits her perfectly.
Literal
She [topic-は] black 's party-dress [subject-が] very well suits.
似合う ('to suit, to look good on, to become someone') is a stative verb describing the harmony between a person and a piece of clothing or color. Critically, the suiter (the dress) is marked by が, not を — 似合う isn't transitive. The structure XはYが似合う is parallel to XはYが好き ('X likes Y') in its topic-subject syntax. パーティードレス combines two loanwords into a compound with no connecting particle — Japanese loanword compounds frequently behave this way, treating both elements as a single unit.