Open Photoshop and create a new document. I used 1920×1200. After that select the background layer and go to Layer>Layer Style>Grandient Overlay. Use #150b06 and # 321c0f for the gradient colors, Radial for Style and Normal for Blend Mode.
Create a new layer and go to Filter>Render>Clouds. Make sure that the colors were black and white for the foreground an background.