Hyperpigmentation. If you have heavy thighs and sweat heavily in that area this is inflamed skin which from a combination of itching and rubbing has developed a thickened skin with dark color. This needs o be diagnosed and treated. The color is difficult to lighten completely and will not respond to over the counter medications.
Itchy thigh skin. There may be a yeast rash called tinea versicolor. Dark, moist and warm skin may foster the growth of normal fungus on the skin. It can cause a slightly darker skin color and scaling. See a board-certified dermatologist to diagnose the problem and treat the condition.
Could be.... Tinea = a fungal skin infection that notoriously likes to grow/live in dark, warm, moist areas - such as you are describing. It can darken the skin and is best treated with keeping the area as dry and cool/aerated as possible, staying away from Hydrocortisone cream (which may lessen the itch - but can help the fungas grow) - should be treated with antifungal creams/powders daily - all are otc.