Focusing on both the CSS and JS at the same time might not be the best approach to take. I would start with the JS as that seems to be your main focus when following the tutorial like you are. I will say without really looking too much at the tutorial code I wasn’t super impressed with it.
Here is another version, I’m not saying the code is great or anything just a different approach. I didn’t really bother to change much of the HTML/CSS and mainly just copied it from the tutorial.