Accordion menu using Vanilla JS, CSS and HTML

I am mostly comfortable with what I wrote. What I found the most difficult is reformatting the website for mobile layouts.

The mobile desktop background image would only kick in once the viewport is less than the width of the mobile image (375 px). This seemed like an abrupt and arbitrary transition. The solution I had in mind was to stretch the mobile background image to fit larger mobile deployments. Would this have been a good idea?

I also found it difficult to control the size of the header where it looks similar in size to the star icon.

I am unfamiliar with handling size styling text and images both). I would appreciate any feedback on my CSS regarding this topic.