How do I create edge fog

how can i do like this :

[attachment=0:wuqx8o14][/attachment:wuqx8o14]

i mean the blue edge fog

...

thanks

Create an image the same size at the viewport. Paint the edges the colour you want and leave the middle transparent. Then place that image on a new layer, above all other layers, and set the Paralax of said layer to 0, 0.