I compiled a test example using gtkmm-3.0 library and I noticed that it looks uglier than every other GTK+ app in my system, as if it ignored my system-wide theme. When I rebuilt that app against gtkmm-2.4, it looks OK.
The code is almost the same, discounting for the Gtk::Application class from gtkmm-3.0 replaced with Gtk::Main from gtkmm-2.4.
What could be the cause of the problem? Why is my app ignoring my system theme when compiled against gtkmm-3.0 but not when I compile it against gtkmm-2.4? Is it some system configuration problem? Or should I do something special in gtkmm-3.0 to use my system theme?
I'm on Gentoo Linux if that matters.