hi，Uncle Agony
我要去参加一个主题派对，不知有什么礼仪需要注意的？我还只是个学生，恐怕买不起特殊的服装啊……

It is only proper etiquette to attend the themed dinner as request i.e. dressed to the the theme.

Themed party is whereby a participant is supposed to dress to the theme. For the bold and adventurous they would go to great length to dress up according to the theme and these people do not require my advice. For the more inhibited either by character or by budget ''improvise" is the secret. If pirates is required either a pirate hat from Toy'r'us or one fashioned from cupboard will do. No need to dress in the full regalia of Captain Jack Sparrow. If Opera is the theme a paper mask and you are Phantom of the Opera. Better still get your friends on the same table involved. Everyone wears t-shirt and skinny jeans and you have "West Side Story". Alternative all ladies on the table wear Qipao and you have " In the mood for love". With creativity the possibility is limitless.