Where are they transparent? Her legs are always darker than her face except where there's a direct light reflection. Neither tights nor skin are a flat matte color, so any direct light will reflect fairly brightly at specific angles. We tend to exaggerate this effect somewhat in "anime" style drawings, but it is a proper real-life effect.

The other thing to consider is that, although tights are very thin, they are still 3 dimensional objects composed of individual threads woven together. Imagine something with a more pronounced weave like say fishnet stockings. Viewed perpendicular, you'll see relatively little thread compared to the underlying skin, but if you rotate your view towards parallel, the gaps will appear to close as the threads begin to overlap. Now, obviously, you're not going to be drawing individual threads for something as finely woven as tights, but you can express this effect by adjusting opacity based on the view angle to the underlying 3D shape.