Hungary has 3 festivals in this guide, spread across 3 cities. They run in July and August, and festival-day temperatures range from 29°C (84°F) to 30°C (86°F), with nights down to 16°C (61°F). That range is the reason there is no single "Hungary festival outfit". The answer changes by month and by site.
Every one of them run in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment.
The ground matters as much as the sky. You will find beach, island and open field, which puts flat sandals you can rinse, plus trainers for the night stages at the top of the list for most of these events. 3 of the 3 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans y2k, costume, camping and beach. Sziget Festival is the largest of them at around 400,000 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.