Tznius comes from within. Two people can wear the same clothing but it looks totally different on both. This is due to how both women project themselves.
If someone needs an excuse to wear a slit e.e the skirt is too long to walk comfortably they know jolly well they should shorten the skirt but they opt not to because they want to wear what they want.
Sometimes a longer skirt can be less tzniusdik than a shorter skirt – again depends on the wearer
There is no problem to dress trendy, fashionable or modern – use whatever words you want but tznius is another matter. It should come naturally.
Any female can becomne a woman. Not everyone can be a lady.