I sadly, and I don’t say this lightly, have to agree.
When first using KDE, I was totally baffled by the amount of themes you can download.
But I quickly learned, that most of them are either more than 5 years old, look ugly/ incoherent, or cause this blurriness issue OP mentioned.
For my taste, Breeze (or to a further extend, Lightly) looks phenomenal by default, especially since Plasma 6. I only change the color scheme a bit (usually just adaptive colors, based on wallpaper), but that’s it.
Definitely! The only “good” 3rd-party themes are the ones from Jomada (Edna, Moe, Itchy, etc.) and from Vinceliuice (Graphite, Canta, Layan, etc.).
Those are well thought out, coherent and look professional.
If you scroll through the top ones, then you almost exclusively find ones that got their last update years ago, and now look extremely incoherent or buggy.
And even if you find newer ones, then they often lack the polish.
Man, it just sucks…
But hey, maybe there is just not that much demand anymore, because the defaults look great nowadays?