I am having trouble seeing what gtknotebook tab is active, so I came up
with some css that ought to do the trick, but does not.

Basically, I want the active tab to be bold.

/* Has a visible effect.  "lighter" doesn't parse.  */
GtkNotebook tab GtkLabel {
  font-weight: 100;
}

/* This works for color; font of sub-widget not bold.  */
GtkNotebook tab:active {
  background-color: #00dd00;
  font-weight: bold;
}

/* This seems to have no effect. */
GtkNotebook tab:active GtkLabel {
  background-color: #0000dd;
  font-weight: Bold;
}