sanya, The indefinite pronoun "everyone" is grammatically singular and takes a singular verb. It requires the singular object in your sentence. Everyone in the operating theatre is wearing a white uniform. correct. The problem is that, although "everyone" is grammatically singular, its meaning is plural, i.e., everyone = every person. So, the sentence--- Everyone in the operating theatre is wearing white uniforms.---also makes sense. This is a weak point of English grammar. You should use the singular if possible, but there are times when the plural would be preferred. EX. -Everyone is in the operating theatre and they are wearing their white uniforms. This sentence sounds better and makes more sense than the following sentence which uses the singular. -Everyone is in the operating theatre and he is wearing his white uniform.

Actually I think should be: "Everyone in the operating theatre is wearing in white smocks". The uniforms have in police or army. But if you want use a word "uniform" you should write: "Everyone in the operating theatre is wearing in white uniforms". "Uniform" is a singular and the "uniforms" is a plural.