Welcome to the Plex-universe. I hope you’ll enjoy it 
Some of the features you describe already exist. For the others there’s already suggestions addressing them.
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You can already place a file called
theme.mp3in your respective movie or tv-show folders which will be picked up when refreshing metadata for those. Plex will then play them while you’re on the details page of that item.
Plex can also be configured to automatically load themes for a lot of shows
→ link to the related support article -
There’s an existing suggestion to support some general background music while browsing the app(s).
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There’s an existing feature suggestion to allow for video backgrounds – I suppose that’s the file you’re looking at. I had been wondering about that some time ago… seeing more blu-ray backgrounds I noticed they’re often mixed with UI components of the disc menu and therefore not always ideal for that purpose (however that’s just my perception)
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There’s an existing feature suggestion to support themes for movie/tv-show collections too
I suggest you comment/vote on those existing threads to help us avoid duplicate suggestions which result in distracted/dispersed votes. This usually prevents individual suggestions from getting the necessary attention.
Therefore we usually try to close duplicate suggestions right away…
