For those who are using Gnome Shell instead of Unitiy.
Ambiance (Radiance) theme has the same problem in Gnome Shell. But unlike Unity, all applications are rendered incorrectly, not only Gnome Calendar. The symptoms are the same: white (or black) top corners instead of transparent corners. Additionally, windows lack shadow.
I have managed to fix this by adding the following lines to /usr/share/themes/Ambiance/gtk-3.0/gtk-widgets.css (or /usr/share/themes/Radiance/gtk-3.0/gtk-widgets.css for Radiance theme)
border-radius: 7px 7px 0 0;
box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.1), 0 8px 8px 0 rgba(0, 0, 0, 0.2);
First line (border-radius) fixes the issue with corners, and the second line (box-shadow) adds a shadow to windows.