I’ll give you an interesting fact: Krita’s current interface style isn’t really the default, so to speak. It has other styles available. As part of KDE, the project has access to other styles, such as Breeze, which was removed due to some problems in the past, but those problems no longer exist (as far as I remember). It’s just a matter of adding it back, something that was already proposed several years ago, which is why I created a thread to have it integrated again.
Even so, if you want to change how Krita looks, you can do so: