cat ~/.config/gtk-3.0/gtk.css

gtk.css

TerminalWindow .notebook {
  padding: 0;
  border-width: 0;
}

TerminalWindow,
TerminalWindow.background {
  background-image: none;
  background-color: #6e6e6e;
  color: #000000;
}

TerminalWindow .notebook tab {
  padding: 5;
  border-image: none;
  border-width: 0;
  background-color: #6e6e6e;
}

TerminalWindow .notebook tab:active {
   padding: 5;
   border-image: none;
   border-width: 0;
   background-color: #d1d1d1;
}