Hi ! studying some gui libraries ( winapi , mfc , wxwidgets ) I really wonder how some programs changed their gui image for forms. overlap ? functions for gui bitmap ? changing embeded gui bitmap? I really don't know how ...

It is generally a matter of how the libraries handle painting of their UI elements.

Your top image paints in solid bold colors, with lines and rounded corners.

Your bottom image paints with gradients, trying to look like 3D elements that bend out to the user.

If you want to draw like this yourself, the common term for it is "skinning".

the common term for it is "skinning".