Open a new file having 675×400 px and 72 dpi painted in black.

After that take the next instrument and represent the lowest layer reserved for the site’s header. Apply the following instrument to stretch out the upper part to the right side.

Blending Options-Gradient Overlay

Gradient’s parameters:

Copy this new made layer 2 times. The first copy should be minimized with Free Transform option, move off the effects from it and change its color into another one – 330002.

on the layers.
The second copy must be minimized too, by using the same Free Transform option and the color of the copy should be changed too into 810100.