Norway has 3 festivals in this guide, spread across 2 cities. They run in June, July and August, and festival-day temperatures range from 20°C (68°F) to 21°C (70°F), with nights down to 12°C (54°F). That range is the reason there is no single "Norway festival outfit". The answer changes by month and by site.
1 carries 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 open field, beach and city park, which puts waterproof boots at the top of the list for most of these events. None of them have camping on site, so you are travelling in and out each day and the outfit has to survive the journey at both ends.
Stylistically the crowd here leans metal, rock, punk and beach. Tons of Rock is the largest of them at around 45,000 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.