Comment 10 for bug 762349

I had the same problem in Oneiric running Xfce with a self-chosen combination of window manager theme and 'appearance' theme.

I found a workaround to be:
- create ~/.config/gtk-3.0/gtk.css
- paste the following in:
  TerminalWindow .notebook tab:active {
        background-color: #def;
  }

This makes my active tab being a bit lighter than the other tabs.

See also /usr/share/themes and look for gtk.css and terminal.css.