It is best to set "Vertical stretching" to "Stretch"
with "Border sizes:"  2,0

There is no way to make the top black line blacker on the "For opacity 33" images as the taskbar works different than the start menu. It's at black 255 alpha already.
I added a slight amount of black to the inside portion of the opacity 33 simple image.
All the images were modified with Pixelformer image editor.  


On the real Win7 taskbar, it is darker behind the start button and under the clock text on the right.
I faked it. It may only work on 1920 width display screens.  
Set "Horizontal stretching" to "Stretch" with "Border sizes:" 138,219
If 0,0 is used, the dark gradient will stretch with the rest of image elongating the dark corners. 



Be aware that sometimes on Win10, the taskbar may not refresh to what is behind it.
Possible solutions:
* Select Opaque Taskbar look option, then switch back to glass.
* If taskbar is unlocked, pull taskbar top up temporarily with the mouse. 
* Select a new desktop background picture. 