https://git.gnome.org/browse/gtk+/tree/gtk/theme/Raleigh/gtk-default.css

has

notebook .active-page {
  color: @selected_fg_color;
  background-color: darker (@bg_color);
}

instead of

.notebook tab:active {
    background-color: darker(@bg_color);                                 
}

Adding the correct version to my ~/.config/gtk-3.0/gtk.css makes it look as it should.

 I don't have a git clone locally to make a commit from, or I'd attach one, but I'm not checking out gtk just for a trivial change like this.

 This has been broken since at least 2011, I guess.
http://unix.stackexchange.com/questions/15510/indistinguishable-active-tab-in-gnome-terminal

 The commit introducing the broken section is
https://git.gnome.org/browse/gtk+/commit/gtk/gtk-default.css?id=7cd3e7c81bf82bc51f2891e332575d1fbe3dde4e
 (3 renames ago)

 Apparently it was broken before that commit, since no lines were removed.  I guess the C code used to have different defaults that didn't need theming for that.

 It's been broken on my system for a long time now, I think, but I finally decided to look into it.  I thought it was because of leftover cruft from Ubuntu upgrades, but yeah, turns out it was just how the default theme was.