SUMMARY
Due to KDE 5.17 new feature
"For instance, we have made the Breeze GTK theme respect your selected color scheme."
I first have made a post in reddit https://www.reddit.com/r/kde/comments/dk8bt2/gtk_theme_broken_on_new_kde_517/
As it seems to be a problem I'm filing this bug report
Comparison showing the problem https://imgur.com/a/eqd9Qjs

Basically the problem is this file (~/.config/gtk-3.0/colors.css) that is changed by KDE when colorscheme is changed
so breeze gtk gets the right colors based on the current kde colorscheme, which is awesome by the way. 
But this might interfere with other gtk themes other
than breeze gtk, like arc gtk for example that uses some variables with the same name as the ones set in that file, 
so the color of the gtk theme is overridden by the current colors of the colorscheme, this should be the case only
to breeze gtk as said in the Release Announcements of 5.17

STEPS TO REPRODUCE
1. Change Colors to dark in settings
2. use Arc gtk theme (light version in this example)
3. Open some gtk application, like gnome calculator where it is visible

OBSERVED RESULT
Gtk theme color is the one from the kde color

EXPECTED RESULT
Gtk theme color be used by gtk applications

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.17
KDE Plasma Version: 5.17.0
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.1
Kernel Version: 5.0.0-32-generic

ADDITIONAL INFORMATION
This problem might occur in other gtk themes as well