There is no such thing as "toning"; looking toned is simply displaying the muscle you have at a given fat level. It is harder for females to gain muscle mass for the reasons that you stated(i.e hormonal), but it doesn't change the fact that the same methods are followed, and the rate of strength adaptation follows a similar curve.
Most people, even the slim looking cardio "gym rat " females, could benefit from gaining some extra muscle mass. At the very least progression through novice levels of strength adaptation (which results in a corresponding gain in mass) is something everyone should strive to achieve. You look better, you're stronger, you feel better and you're healthier. Perhaps more relevant to this story, having more lean body mass means that your basal metabolic rate is higher and you burn more calories just by carrying the muscle than you would without.
Mark Rippetoe and Lon Kilgore have written extensively on this subject, and reading anything by them is extremely valuable in understanding strength training and it's benefits. I highly recommend Starting Strength (author's website here
though some preliminary information sans physiological elaborations can be found on a user made wiki here
) for everyone to read. It covers the physiological basis of strength training and a very straightforward way to approach it.