How to stop Gradient pattern background from showing through the black background

How do I fix the code so that the gradient background can't be seen through the black background?

To test code click Run, not update.

code: https://jsitor.com/uMjDlt3E8

These viewports can be set inside the JSitor code.

Small Desktop (1280 x 1024)

Medium Desktop (1920 x 1080)

Big Desktop (2560 x 1440)

iPad Pro (1366 x 1024)

Is there a way to fix that?

.curtain {
  flex: 1 0 0;
  margin: auto;
  max-width: 642px;
  /*box-shadow: inset 0 -2px 0px 0px #0a0a0a;*/
  border: 20px solid #000;
  border-radius: 3.2px;
  border-color: #000 #101010 #000 #101010;
  background: radial-gradient(circle, transparent 0% 35%, #0a0a0a 35%);
}