Plasma doesnt touch the ~/.gtkrc-2.0 file anymore.
I will leave here mine so you can use it to create or edit your own:
gtk-cursor-theme-size=32
gtk-enable-animations=0
# File created by KDE Gtk Config # This is a lie now ;-)
# Configs for GTK2 programs
gtk-font-name="Noto Sans, 10"
gtk-theme-name="Breath-Dark"
gtk-icon-theme-name="Papirus-Adapta-Nokto-Maia"
gtk-fallback-icon-theme="breeze-dark"
gtk-cursor-theme-name="volantes_light_cursors"
gtk-toolbar-style=3
gtk-menu-images=1
gtk-button-images=1
gtk-primary-button-warps-slider=0
(this may or may not apply to your other examples … but its the fix for gtk2 theming, like gcolor2)