I'll let the screenshots tell the beginning of the story.
Gtk+3 app (GDebi) is in the lower left corner, Gtk+2 app (Caja) is in the upper right one. The system is VirtualBox with freshly installed Mint 13 MATE edition.
This is Mint-X:
And this is Mint-Z:
As you can see, these apps look differently with Mint-X, but almost the same with Mint-Z.
Now there's the problem (with me) - I don't like Mint-Z (with these pointy buttons and a strangely enlarged font), I want to use the good old Mint-X, but I also want Gtk+3 apps to look identically to Gtk+2 ones.
Well, I couldn't find a way to achieve this.
I've tried messing with dconf-editor, gconf-editor and even lxappearance - no good. Later I've found that /usr/share/themes/Mint-X/gtk-3.0 subdirectory is identical to /usr/share/themes/Mint-Z/gtk-3.0 one - apparently, someone just copied it there. That's why Gtk+3 application looks exactly the same with both themes.
Well, apparently I have to hack in Mint-X/gtk-3.0 subdirectory. I need help with understanding where to look for the mentioned parameters - font type, widget shape, etc etc. I can't just copy things from the gtk-2.0 subdirectory since the syntax is totally different and I don't know anything about CSS.
Please tell me if you have any ideas.