Well, the error message should be obvious: you have incompatible packages. Right now you have gnome-themes-extra installed yet gnome-themes-standard is incompatible with it. You can start by removing the extras package to see if that works (using pkg delete).
Just curious though: why build all this from source in the first place? It might be a lot easier on yourself if you simply use binary packages: # pkg install gnome3. Be careful though that you don't mix up ports & packages, if you go this route you'd have to install everything as packages first using: # pkg upgrade -f.