hsetroot is a tool which allows you to compose
wallpapers ("root pixmaps") for X. It has a lot
of options like rendering gradients, solids,
images but it also allows you to perform
manipulations on those things, or chain them
together.

You could use one standard background image for
instance, and using tint to make it fit your
current theme. It is compatible with semi
translucent applications like aterm and xchat.