PayPal payment site - old and new design - where to set?

Hello,

when I pay using PayPal on eBay, I always see the modern style site.
In my Magento store I use PayPal too, but in this case the old payment site is shown. But when I switch to Sandbox-Mode, my shop shows the modern payment site too. What does it depend on which style PayPal loads?

For better explanation, see the attached image.
On the left you see the old design on the right the modern one.
Paypal-payment-site-design.png