https://thelibre.news/the-future-of-kde-styling-and-design/
This article explains some of the reasons why KDE sucks UI-wise. The tl;dr is that KDE is a Windows-tier clusterfuck of implementations that do the same thing that make having a cohesive design difficult.

>Firstly, we have Plasma Styles. These only apply to Plasma, your shell, and everything else completely ignores it.

>They are implemented via SVG files, which have benefits and drawbacks. Mostly drawbacks. Sure, they might be very easy to make, lots of people know how to use SVG, but they are very hard to maintain, they not very fast - because you have to render out entire images - and there's no easy way to expose configuration options.

>Which is why you will see the SVGs to be full of these little colorful blocks that are not actually displayed anywhere, but the parser checks whether they exist or not to know whether to turn on and off a certain flag. It's a bit hacky and overall I don't like it.

>Next up we have Application Styles. They are directly used by all QtWidget applications, which is the type of UI toolkit used by apps such as Dolphin and Gwenview.

>Application Styles are written in C++ and have to be compiled to be used. The only way to get them is either by, well, compiling them yourself, or if they're shipped in your preferred package manager. On top of that, they're extremely, extremely difficult to make. There are a few third party projects out there who try to branch off Breeze, but starting from scratch is not something you can just do.

>To repeat: Application Styles, which are actually called QStyles, apply on QWidget applications; whereas QtQuick Control styles apply to QML applications.

>Kvantum is an Application Style which - to put it very simply - converts a SVG file to a style. Basically, it draws an SVG on the screen. You're again missing out on performance by choosing this, but it does make it much, much easier to create custom Application styles.