Breakdown of kanojo wa shouri o yorokobimashita.
Questions & Answers about kanojo wa shouri o yorokobimashita.
は marks the topic of the sentence. It gives a sense like as for her, ...
So 彼女は勝利を喜びました is structurally like:
- As for her, she rejoiced at the victory.
If you used が instead, it would focus more directly on she as the subject, often with a more identifying or contrastive feeling, such as she is the one who rejoiced.
In a simple, neutral statement like this, は is very natural.
