Her dress was riotously decorated with sparkling gems and intricate embroidery.

riotously 是副词，由形容词 riotous 加副词后缀 -ly 构成。riotous 来自名词 riot（骚乱、狂欢），引申为"放纵的、喧闹的"，在这里形容装饰风格极其夸张华丽、充满视觉冲击力。riotously 修饰 decorated，表示"以极其张扬炫目的方式装饰"。