This is a background of a soon to be published WindowMaker theme titled as Fade-effect. Even though this will be a part of a WindowMaker theme there is no reason why it can not and should not be used with other environments. Enjoy and have a great day.
----
If you are a Firefox user take a look at a persona made to complete this artwork:

https://addons.mozilla.org/fi/firefox/addon/fade-effect/