The design for the room was of the fancy style popular in thise days. 屋子的设计是当时流行的极富艺术表现力的建筑式样。

be of +名词（词组）: 表主语的某种形式或特征

I am pleased to have been of help to you. 我很高兴对你有帮助。 All the students in the class are of the same age. 这个班上学生年龄都一样。

类似的用法还有： of great difference=very different of great importance= very important of no use=useless of little value= not very valuable