The Part Design Thickness tool removes the selected faces from a solid and adds thickness to the remaining faces.
The tool can be applied to a basic box to hollow it out, or to a cylinder to turn it into a pipe.
Making a Box
Make certain you have changed to the part design workbench.
Also check to make certain the “Body” containing this cube is active.
Select one side of your cube
Then click the Thickness icon from the Part Design Workbench
The face you selected will be removed and the “Thickness Parameters” dialog will appear in the Tasks Tab.
There are a number of paramaters. The thickness value determines the amount of offset that will be added between the existing faces and the new faces.
You can use make thickness inwards, if you need the size of the cube to remain the same.
When you are happy with the result of the Thickness click OK.