。
She got dressed up to go out to the party.
Literal
She [topic-は] party [to-に] go-out [in order to-ために] dressed-up.
盛装する (せいそうする) literally means 'abundant-dress' — to dress in one's finest, to wear formal or showy attire suited to a special occasion. It's distinct from 着飾る (to deck oneself out) and ドレスアップする (the loanword), each with subtle shades. ~ために here takes a verbal antecedent — 出かけるために, 'in order to go out' — which is the volitional purpose sense, requiring the same actor in both clauses. The compound verb 出かける ('to go out, leave home') is a fused 出る ('exit') + 掛ける whose etymology has worn smooth over time.