I am using the standard. I can see the difference: flat is intentional. As per design specs. I refer you to the raspberrypi.org forums (and blogs!).
In this case Lazarus adheres and other programs don't, but I may be wrong until I see sufficient evidence.
I simply don't get it.
I have tried to make a very simple c program, using GTK2, and that behaves as intended (#include <gtk/gtk.h>, and compiling with GTK2+), meaning the button has rounded corners, and color gradient (see the left side of the image below).
However compiling the Lazarus application with Widget type set to GTK2, the result is still flat, as shown on the right side of the image.
So to me it seem the other way around, that Lazarus simply does not adhere.
To be sure you understand this: Lazarus doesn't render that, the default rendering engine from the distribution does that. Lazarus uses the rendering engine of the OS (debian derived Raspbian).
Note that is NOT lxde, but a modified version of it.
Yes, but Pixel is stil a fork of LXDE, and is installed by default as LXDE, same naming.