I cannot draw, but I can write some code to make some idea into something visual.
When you illuminate a cube from the inside, the corners and edges naturally appear a little darker, and the middle of each face a little brighter. I assume this is a consequence of how the cube was constructed, with the material being thicker at the edges and thinner in the middle.
HTML and CSS doesn't provide a true 3D environment. You can't make real three dimensional shapes but you can move 2D shapes in a 3D space. You can make a cube by making six squares and positioning them edge to edge and by applying an inset shadow to each face, you can make it look like a cube with that illumination effect.
It's on CodePen.