Link to the said theme will help us to help you.
Windows 7 in full screen has full grass, but Windows Vista when you maximize window, the glass becomes black and opaque. I'm remember i must to edit one HEX value in .msstyles to enable full Aero Glass. Unfortunately, those who created these themes for Window Blinds have the same bug.
https://drive.google.com/file/d/1JgXMUBqS42CeTKVo9WZDenGbMF6r8BGo/view?usp=sharing