I mean that PCManFM and Thunar follow my theme:
Nautilus does not:
I have to add that I do not want to change my present theme. And I guess that is the problem, as it does not have a gtk3 folder included.
Nautilus is gtk3. Thunar is gtk2. I don't know about the third. Ensure that the theme you use has a gtk-3.0 folder if you want Nautilus to be styled as well.
Your theme may be in
The presence of the gtk-3.0 folder ensures that apps based on gtk3 will be styled correctly.
Edit: I suspect that most software from GNOME will be gtk3. Other things like Chrome, Firefox, LibreOffice still are gtk2.
Anyway, as the other answer also suggested:
Nautilus is gtk3: use a gtk3 theme if you want a gtk3 program to be themed.
(In order to use a theme that does not have gtk3 support: a not very recommendable workaround is to copy the gtk3 folder from a theme that has it into the folder of the one that does not. In fact, in Lubuntu, one can extract and copy themes in home/username/.themes in order to install them, a location which does not require root privilege. But some applications like Synaptic Manager require a theme in
Response to latest updates of requirements by OP
If you don't want to change your theme, I fear it is not possible to workaround this. Because, nautilus version 3.0 or greater requires gtk-3 themes, which your currently active theme surely is not supporting.