I am really happy about KDE's default theme - Air. It looks fine for me, and I don't want to change it.
I also never have desktop effects enabled on KDE (and on any DE) as I never need it. However, every time I disable the desktop effects on my KDE system, I notice that the Air theme gets too dark which is unpleasant, unaesthetic and simply makes the letters on, say, the Task Manager hardly discernible.
I really wanted to get the light grey color of the theme back. So I've notice the following manipulation usually helps get the theme back to being light-color:
System settings -> Desktop effects -> Advanced - then switch between the Compositing types and QT graphics systems until you notice your Air theme stops being dark and gets light-grey.
The behavior of those settings is a bit unpredictable for me, as various options have brought in the same result (right now I use OpenGL / Native, and I have the light theme, but I remember I've used other options in the Advanced tab on my other KDE installations before to get the theme light). Anyway, the good thing is that switching between those options does work.