When the GNOME staff has decided to hide the menu icons they also removed the stock icons in the buttons, uniforming the user interface and making it a bit smaller.
The good news is that is still possible to enable and show the buttons icons again. The above image shows the GNOME Screenshot dialog with the buttons icons enabled.
To show again the buttons icons you could execute the following command in a terminal window:
gsettings set org.gnome.desktop.interface buttons-have-icons true
The change is immediately applied and doesn’t require any restart.
To restore the default look by hiding the buttons icons you could simply execute the following command:
gsettings set org.gnome.desktop.interface buttons-have-icons false