Now KDE does provide compatible themes for GTK, but the fragmentation in GTK (2 vs 3) does not help. The solutions are at best a stopgap measure till the next major version breaks what you managed to get right. It's a never ending game of whack-a-mole. It does not help some of the most popular apps are written in GTK. As I speak due to reasons I haven't investigated yet, I am unable to paste text contents in GTK apps. Windows has other points I take issue with, but at least the UI just works as expected and doesn't seem to hang by a thread.