It looks great! Apart from the sizing it is looking super close to the original design.
To maybe fix the sizing I would look at removing the div blue-area-behind and instead applying the blue background colour directly to the body. If you then make the body display property flex hopefully you will be a bit closer to the original design.
It's worth noting that children of elements that display as flex automatically become flex-items so should shouldn't need to set all of your elements to display flex.
When I was first learning flexbox this course by Wes Bos really helped me out.
https://flexbox.io/
Really good job! Keep it up 👍