Incomplete instructions on "Setting up GTK for Windows"
Edit: The title of the section I'm talking about is: Building and distributing your application. But on closer inspection it's just about putting Adwaita and hicolor icons in your app directory, of course if you just do this your app won't work and you need the necessary MSYS2 MinGW64 DLLs and various other files. Also if you do have the entire MSYS2 install you will have the hicolor and Adwaita icons working anyway. This entire section is of very little use.
https://www.gtk.org/docs/installations/windows
I like that the GTK project recommends MSYS2 for windows but the instructions are a bit confusing.
Under "Building and distributing your application" it says
Step 1. Copy the gtk-3.20 folder of that repository to a folder share/themes/Windows10/gtk-3.0/ in your installation folder
But not where the 'gtk-3.20' folder is. I assume it's somewhere under C:\msys2\mingw64 but cannot find it. I also assume it should be the 'gtk-3.24' folder now and that may change again.