There's no shade, the blank space between "Atom" and "Upload file" is a completely solid, same color from one side to the other. Hence the reason it's an illusion, it just looks like it's shaded.
There's no shade option in HTML or CSS
. When you view a page it's combination of assigning colors and images if you want anything like shade. For example main navigation bars on the top are created with three images.:
This is set as the background and it's just repeated:
This is a single image used for both the top and bottom corners,
If you look clsoely at the corners you can see it as it's just a little too light on the top bar and a little too dark on the bottom bar.