I’ve done 20px to exaggerate things so you can see the effect. Adjust the 20px values as you desire. The last bit of code radiuses the top of the header area where the grey top border is and you can see it fades into nothing at the ends.
border-radius: 20px 20px 20px 20px;
border-radius: 0 0 20px 20px;
border-radius: 20px 20px 0 0;
If you want to get rid of the top grey border line, change the third bit of code I gave above to the following.