Change over time is something different from apps looking vastly different at any given time.

Unfortunately nowadays even the built-in apps on the major desktop OSes are inconsistent, so the temptation for third-party apps not to care is somewhat understandable.

I change clothes all the time too, still match the pieces of clothing each time.

There's aesthetic value to coherence. There's also design, usability value. I have Telegram, Steam and Firefox opened right now and each one of them displays different minimize/maximize/close buttons on the top right. That's not ideal.

So if you already wear clown shoes does that mean that you have to wear only clown costumes?

Someone who is in full clown costume looks less ridiculous than someone who wears clown shoes and is otherwise dressed "normally", yes.