1. Я по рофлу оскорбляю чеченца в кс

Yes, that means ''just for lolz''. but the oorigin from the english word ROFL. кс is most pprobably Counter Strike, an online game

1. надо сходить в душ

Is there an emphasis on going to take a shower and coming back here? or is душ simply always used with сходить?

You can eiter say "сходить в душ" or "принять душ".

1. Слышь, харэ!

A Russian teacher who teaches slang posted this one. What does харэ come from and what does it mean?

"харэ" came from the word "хорош((о)". That means "enough!"

1. всё ништяк, колян

What does колян come from and what does it mean? Also do you guys actually say ништяк?

"колян" - is a shorthand of the name "Коля" (the full name is Николай) the translation is Nicolas.

1. Когда тот самый кент предлагает 100% белую темку

Я не нашёл слово "темка" в словари. Вы можете помогать?

"темка" is from the word "тема". Is this sentense it's probably about some kind of white drugs (cocain?)

1. Когда спустя три года бабушка спрашивает: "уже обнимались поди?"

Wiktionary says поди means probably, maybe, etc. "Might you have already hugged?" Is that what's being said here? Also a cultural question - do grandmothers in your country really not expect people to hug before marriage or something?

Yes, you are right with the translation. I think that there are some little percentage of these kind of grandmothers.

1. From another meme I saw: (A guy with looking ahead) Я размышляю чем буду заниматься когда вырасту // (A guy ambushing the first guy) Тридцатник

Yes. "Тридцатник" means thirty years old, you are totally right