Why do dark skins think that light skins are more superior than them?

Not everyone thinks that way.

Probably because a lot of them have been raised with their parents telling them "don't go in the sun, you'll get darker" and that they have "bad hair" and ugly features.

At this point it doesn't make sense to blame "white beauty standards". Most of this is perpetuated by black people themselves, not by white or mixed race people. It's a cycle they have to break in raising their kids with self-confidence, not society.

India is a pretty messed-up place in many ways. This is one of them.

Prejudice can go the other way as well. Check out what the Nation of Islam believes about "white devils".

Family.

I can't speak for India or other parts of the world, but in America, prejudice and racism are taught. Older generations that are still alive to tell their tale of slavery, segregation, and more, like to pass their hate onto their family. Even now, I have a friend who grandmother told her to straighten her nappy hair so she can be more appealing and accepted; though it's not like that nowadays, in times of segregation and extreme racism, looking like your predator was a way to stay safe and make you seem more welcoming.

Maybe they are not wrong with everything.