Greece is similar to the rest of Europe in terms of fashion and dress standards. The fashion is very different from Canada or the US. I'm not sure where you're from. I would say wear what you are comfortable with. Greece gets very hot, and if you're going to be visiting a lot of sites I would say wear light and breathable clothing.
One of my favorite memories of traveling to Greece is visiting the outdoor movie theaters. It’s such a unique thing to do, I haven’t seen it much in other countries. It should actually become more popular because it’s so covid safe!