transparent bkg layers on top

In several of the themes I've wanted to use, there seems to be an opaque layer over everything.
In another few there will be a small rectangle of the background on top of everything.
It's semi transparent.

I've been setting up a desktop, and now the current one has developed this same artifact.
I've looked in the object list for a layer, but there isn't anything there.

In the gridscape theme, I get two of these that are constantly scrolling words very fast.
That is also where I get the opaque layer over everything.

I am currently working with Saturn grey.
Even as I speak, the rectangle in Saturn Grey has disappered, but I'd still like to know what is going on.
And what's the problem with Gridscape

I thank you in advance.