HTML + CSS + JS for manipulating CSS

What are you most proud of, and what would you do differently next time?

This was my first use of JS on my very own, which caused a bit of a struggle, but finishing it with desired effect is a great reward :)

What challenges did you encounter, and how did you overcome them?

implementing basic JS for DOM manipulation. As first time without any guidance I had to revisit documentation quite a few times and use stackoverflow for inspiration and answers.

What specific areas of your project would you like help with?

I have a strong feeling that my code might be clumsy at this point. If there is anything anyone notices that might be done in an easier, more elegant way, I'd love to get some feedback!