Base Apparel Page using HTML CSS and JS (grid & absolute position)

Hello, I've struggled a lot with this challenge. I have a few specific questions but also welcome any other feedbacks.

1- I use setTimeout so that the form is clear before the alert pops up, will there be any issue? If I use promise instead, how can I do this? Any other way that you guys did to clear out the form after successfully submitted?

2- I learned from someone else how to do this linear background but I'm still very confused about how to achieve it. Is there any good tutorial on this topic that you'd recommend? How do you just look at this background design and know which colors to use for the linear gradient?

This is my second project using JS. Your feedbacks will help me learn a lot so I hope to receive some comments on this project. Thank you in advance :D