She doesn't sound especially thin - or at least, you *should* be able to see the last couple of sets of ribs on a boxer. If they've disappeared under flesh, the dog is actually overweight

It's a lean breed we've got here - muscle on frame, not well covered like a labrador.
If it's more than just the last couple of sets of ribs though, you might consider increasing the amount of red meat versus things like chicken or fish in her diet. This is still protein (not carbs or fat) so should help to build lean muscle mass as opposed to just covering things up with a layer of unneccessary fat (that you'll probably regret later

).