In recent iterations of GNOME terminal, the active tab is nearly indistinguishable from the inactive ones.  That makes it harder to navigate when you’ve got a bunch of terminals open simultaneously. Fortunately, GNOME uses a modified CSS scheme to control theme appearance, and that makes it easy to highlight an active tab. Here’s how.

Create (or edit) ~/.config/gtk-3.0/gtk.css. Add these lines:

@define-color ubuntu_orange #fb9267;
TerminalWindow .notebook tab:active {
 background-color: shade(@ubuntu_orange,1.1);

Then, exit all open terminals, and then open a new terminal and create a tab. With the above modification, you should see tabs like this:

GNOME terminal tabs