First of all, I know that this issue has been raised a few times on Stack Overflow the past couple of years. The problem is that sometimes I get the new PayPal checkout pages and sometimes I get the old pages.
I've made two test accounts in my sandbox. The first one got the new design, and the second got the old design. My live account seems to be using the old design. It may seem that US accounts get the new design, and foreign accounts get the old design, but I'm not sure about this.
See for example these questions from 2014
The answer earlier has been that the new design is slowly being rolled out, and that it is randomly selected.
Now, my question is if anyone knows if this is still the case, and if/when we will be able to select the new design?