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I'm still struggling with responsiveness. Is it done well? What can I improve?

I'm not sure about responsiveness and my flex skills. Can I do something better? Is there any better way to set the button text color (make it to take color from the parent of the parent via css)?

1. Is that responsive enough?
2. Code refactoring - what can I do better?
3. Are my JS function well written? Can I do something better?
4. I decided to add classes (with new CSS style) of components for page transition and change the appearance of clicked button. Is that a good solution? Are there any other options (except of adding inline HTML style)?