Thanks for your input Felixbone, I changed the BG colour to the bottom gradient colour and changed my post to reflect.
The OP asked how to make the background so I did, following my instructions, will bring my result as seen in my attachment.
The last colour at the bottom is #3F65A3 and then gradients up into #020202 which is all I could go off from the image that was posted.
Changing the image to repeat-x will make no difference unless your forum is longer that 1350px which is why the background colour should be #3F65A3. If it does indeed become longer then you will not see the change as all the gradient is set at the top and not at the bottom.
Happy designing