France has 10 festivals in this guide, spread across 9 cities. They run in February, March, May, June, July and August, and festival-day temperatures range from 3°C (37°F) to 28°C (82°F), with nights down to -6°C (21°F). That range is the reason there is no single "France festival outfit". The answer changes by month and by site.
1 of them runs in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment. 2 sit below 16°C, where the outer layer effectively is the outfit. 2 carry a high rain risk, which makes waterproof footwear a requirement rather than a precaution.
The ground matters as much as the sky. You will find city streets, snow, industrial site, open field, forest and city park, which puts waterproof boots at the top of the list for most of these events. 4 of the 10 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans indie, streetwear, elegant and camping. Nice Carnival is the largest of them at around 200,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.