Scaling of icons does not work for gtkmm applications

I have a 4K display, and the correct scaling would be 3x. I have put Xft.dpi: 300 in .Xresources and GDK_SCALE=3 GDK_DPI_SCALE=-1 in /etc/environment. This can scale most applications, but the C++ applications that use gtkmm do not scale. Some apps are Nitrogen and GParted. Using this method, the text scales but not the icons. How do I scale the icons for gtkmm applications?