Calculator App using CSS Flexbox and CSS Grid

This was my very first Intermediate challenge on this site and I have to say, it was very stressful. I now understand that as the level for each challenge increases, the number of components and styles increases (In other words, lots of work). So I attempted it and unlike the other challenges I've completed, it took a lot of time for me to complete it and so I'd really appreciate if you could leave me a very honest feedback cos I'd like to know whether all the time I spent on this challenge was worth it. And also, I'd also welcome some helpful tips on how I could improve the solution, especially the JavaScript (which is messy by the way)