backface-visibility in 3D cube

[fglez94]

Issue: Applying a cube rounded corners, the content behind is shown.

What I want: Perform a cube with rounded corners, create the effect, or even better, perform a cube with filled content.

What I need: To fill the whole content inside the cube, make the cube looks like is filled.

https://codepen.io/fglez94/pen/WNjBXxd