OK I played around with it a bit more to troubleshoot.
I added the feature to a post. I found several things:
* the diagonal feature works when slanting from right to left, but does not display at all when slanting from left to right (regardless of whether it is the top or bottom).
* Even when the margins and padding were set to zero, there is still a line above the diagonal that is not included in it, such that there’s a small border at the top of the diagonal, which is really the image showing through underneath.
* There was also a slight one-pixel border on the left even when margins were set to zero.
* I’m not sure what the defaults are meant to be, but I am surprised that the default width is 50%. Shouldn’t it be full-width?
* Perhaps related, when I set the margins and padding to custom, it won’t let me reduce the numbers lower than 10 pixels. And when I tried, it reverted my entire theme back to the defaults! All the colors suddenly changed back from the new global settings to what had come with the theme. So I take it that is a bug. It seems this only affected the global style settings because the structural change I had made to the template header persists.
Any update on this? I recently wanted to try another of your plugins (the one for backgrounds). I ran into some technical difficulties (the parallax background layer that should have been on bottom was on top) which I would gladly have reported to you but it seems you do not read your support posts. =[