I wouldn’t expect anyone to waste even 5 minutes of their time implementing themes and gimmickery of that nature, but I’d be willing to listen to ideas about it - if any real reasons could be presented, that is.
Desktop computers are for doing things. Things like email and other online communications media; composing music; numerical calculations, etc. The operating system is there to support that, in various ways, particularly a file system and a variety of user interface conveniences.
Where the UI can be improved in terms of better support for the things we do on the computer, then we’re talking about real improvement. A simply more glamorous UI appearance, on the other hand, is an absurd and futile pursuit, the kind of thing that gets us nowhere and is never good enough for the people who clamor for it anyway.