The shadow is relative with visual-style file(*.msstyles).Use Aero 8 by xxiNightxx theme.
And if I choose "rounded corners" theme, there are some artifacts on the corners.
You forgot to do this. With this W8 won't fill up the corners with color.
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\DWM:RoundRectRadius (DWORD, Win8 = 0, Win7 = 12)
With my custom theme I have found this reg setting that will enable rounded corners
and UserPreferencesMask and TranscodedImageCache...DP