GTK-приложения не используют Oxygen-gtk.

cat ~/.gtkrc-2.0
cat ~/.config/gtk-3.0/settings.ini

debasher@deb:~$ cat ~/.config/gtk-3.0/settings.ini 
[Settings] 
gtk-theme-name=oxygen-gtk
gtk-icon-theme-name=oxygen
gtk-font-name=DejaVu Sans 10
gtk-cursor-theme-name=oxy-obsidian
gtk-cursor-theme-size=0
gtk-toolbar-style=GTK_TOOLBAR_BOTH_HORIZ
gtk-toolbar-icon-size=GTK_ICON_SIZE_LARGE_TOOLBAR
gtk-button-images=1
gtk-menu-images=1
gtk-enable-event-sounds=0
gtk-enable-input-feedback-sounds=0
gtk-xft-antialias=1
gtk-xft-hinting=1
gtk-xft-hintstyle=hintfull
gtk-xft-rgba=rgb
debasher@deb:~$ cat ~/.gtkrc-2.0 
# DO NOT EDIT! This file will be overwritten by LXAppearance.
# Any customization should be done in ~/.gtkrc-2.0.mine instead.

gtk-theme-name="oxygen-gtk"
gtk-icon-theme-name="oxygen"
gtk-font-name="DejaVu Sans 10"
gtk-cursor-theme-name="oxy-obsidian"
gtk-cursor-theme-size=0
gtk-toolbar-style=GTK_TOOLBAR_BOTH_HORIZ
gtk-toolbar-icon-size=GTK_ICON_SIZE_LARGE_TOOLBAR
gtk-button-images=1
gtk-menu-images=1
gtk-enable-event-sounds=0
gtk-enable-input-feedback-sounds=0
gtk-xft-antialias=1
gtk-xft-hinting=1
gtk-xft-hintstyle="hintfull"
gtk-xft-rgba="rgb"
include "/home/debasher/.gtkrc-2.0.mine"