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 isntance,
and using tint to make it fit your current theme. And yes, of course it
is compatible with semi-translucent applications like aterm and xchat.

This requires imlib2.