It’s in a major key and has been used ‘in-universe’ just like this in Rebels, for what it’s worth.
ImperialFighter said:
…the large monitor in the background of that busy area which is showing Imperial ‘recruitment’ footage of it’s fleet…is playing John William’s EMPIRE STRIKES BACK ‘Imperial March’ music to accompany it!
In both the Old and New EU, the Imperial March is the state anthem of the Empire, and the Main Theme is the state anthem of the Old Republic.