Switzerland has 4 festivals in this guide, spread across 4 cities. They run in April, July and August, and festival-day temperatures range from 8°C (46°F) to 26°C (79°F), with nights down to -2°C (28°F). That range is the reason there is no single "Switzerland festival outfit". The answer changes by month and by site.
1 sits below 16°C, where the outer layer effectively is the outfit.
The ground matters as much as the sky. You will find snow, city streets and open field, which puts insulated waterproof snow boots at the top of the list for most of these events. 1 of the 4 has camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans elegant, snow, techno and apres-ski. Zurich Street Parade is the largest of them at around 900,000 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.