The most important advice is to wear comfortable shoes. You'll be walking a lot and the Roman cobbles are seriously hard. I note what fromparis says about trainers, but better a pair of Converse type shoes - but preferably with thick soles - or even trainers than feet cut to shreds so you can't walk anywhere. I only wear ballerinas in the evening - there just isn't enough support to the foot or thickness of sole for the day. Flat boots should be fine as long as they have nice thick soles.
The truth is, though, that the place is full of tourists and you are unlikely to be mistaken for an Italian unless you already have very Italian colouring and the wardrobe to match. So it's up to you really - I'd wear what you feel comfortable in and what you'd wear on non-work days in any city.
I go to Rome a lot and take what I'd wear on smart casual days in London, where I live. That tends to be unfussy, reasonably stylish clothes anyway - but I'll never look like a Roman!
Dresses are good - but avoid flowery ones (which I don't like anyway except perhaps in Provencal villages, or beach resorts) if you want to fit in with local tastes. Black and white, separately or together are always good, as well as being in style at present.
At that time, you'll need a lightish trench or parka as well as a jacket as it can rain hard.
Dona Ana lives in Rome I believe, so can certainly tell you what stylish Romans are wearing.