The period of this movie was in 2004, I can tell this because when it shows the name of the main actor/character on a student ID card it says '2004' underneath. I then found out than in an interview with the director the although the clothes and styles in the film look like they are from the 80s, as the movie is set in a small town the characters are behind in the latest styles, but it is set in 2004.
They use pastel and very plain colours for the background and the type of food they use. The props used are the plates of food and pieces of paper that are related to the rest of the film, each time these appear they show names of the actors or someone who had been associated with the film on screen or behind it.
The music they use in the background is continuous beat of a guitar and the same monotone voice of a male singing. There are no sudden changes to the music, for example the voice or beat doesn't go any higher or lower throughout the opening. There is no talking in any of the opening so it doesn't explain the start of the film and they only show one picture of the main character.
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http://www.artofthetitle.com/title/napoleon-dynamite/