Open a new file with
766x400 px and
72 dpi. It must be painted after that with this shown color –
424242.

Take now this tool

to represent the lowest layer of the central site’s panel.
Fill 0% on the layers.
Blending Options-Drop Shadow
Blending Options-Gradient Overlay
Gradient’s parameters: 
Take now an appropriate fragment of the picture and place it exactly the same way you may see in the following picture.