I'm not really sure what you want to do exactly.
If you just want to get rid of the dark edges then if your frames have a solid color just make them all one color.
Here is an illustration. I have "Show Shadows Under Windows" unchecked and I'm using Shadowless top and bottom frames. Looks like there's no frame edges:
If your frames have transparency it gets more complicated but can be done.
Hard to answer you without more information and maybe a picture of what you want exactly
