Comment by groks

  cat << EOF > ~/.config/gtk-3.0/gtk.css

  /* Make window title bars more compact.
   *
   * From: https://unix.stackexchange.com/questions/276951
   */

  headerbar entry,
  headerbar spinbutton,
  headerbar button,
  headerbar separator {
      margin-top: 2px; /* same as headerbar side padding for nicer proportions */
      margin-bottom: 2px;
  }

  EOF