What isn't appropriate is too much skin: e.g. shoulders, midriffs, legs. Short shorts are definitely out and we'd personally feel uncomfortable even in Bermuda shorts.
What most of us wear for sightseeing is usually acceptable.
Do bear in mind that the basilicas of Rome, particularly, are important places of worship revered by hundreds of millions. Of course any religious building is a place of worship, and we should show respect for its congregation.


