i have made the challenge with the help of html and css flex

Hii , first i made desktop version which was not mobile responsive, then i made mobile version which was not desktop responsive, atlast, with my sherlock mind i made this step by step and it is working fine.

My code must be very messy, though any feedback is appreciated, please go and have a look in to my code, any guidance will be appreciated. Should i dive into javascript chapters or should i learn more css ?

Thank you.