and configure all by recommendations
but oxygen-gtk theme does not affect the appearance
I have posted a complement to the question you linked to.
This application on the first screenshot is probably using GTK3, while the original instructions only apply to GTK2. I posted over there a link to a site that gives instructions for fixing GTK3. I'm not posting here too because I'm not sure if this would be considered duplication.
Now that you have gtk3-engines-oxygen installed you need to do another step; inside ~/.config/ create a new directory with gtk-3.0 as it's name, then inside that directory create a new file, name it settings.ini inside that file put this:
Save and close, then you must be able to see gtk applications very well integrated with oxygen theme. If not, open systemsettings again, go to GTK appearance and reselect the oxyge-gtk theme, save settings. Now you should be able to see good looking GTK applications.
This worked for me as far I compiled the oxygen-gtk theme from it's lastest tarball, maybe you won't need that but if the latter isn't working for you, you should consider compiling.
Hope this helps! Good Luck!