I guess the people who use KDE don't maybe care about this sort of polish enough, which is why it doesn't get fixed?
The desktop/windowing system for me is just a means to launch an application which is where I spend most of my time. This often happens to be a browser, a terminal and IDE etc. The design of such apps takes more precedence for me than some margin of the control center items where I couldn't recognize what the problems were if you asked me to.
What bothers me more than the visual design is the interaction design ... am trying to find a way to move a sticky note which has been on my desktop for quite some time and the method I used previously no long works. Also some releases ago they mapped 'Alt-F' on the Konsole terminal app to a Find input box, whereas the binding is normally used on terminals for "move cursor forward by a word". This is not a sane default and has caused me to pick Wezterm as my preferred terminal app.
Like, it looks "bad," but it's honestly not that bad.
I do use third party themes but I suspect they have the same problems. I just have some colour preferences and like pretty icons.
I prefer to have third party apps icons overridden by the icon theme, but then I can just use another icon theme.