Like in gnome Shell with SSD windows. Using normal box shadows and some simple Code ist should be possible to port almost all themes to scss window borders. This would improve compatibility fir CSD Apps and make the system Design more consistent. Im not talking about dropping SSD windows. Just style them Like the CSD ones using SCSS Code. This way CSD and SSD windows would look more similar.
gtk widget factory is a csd application the rest is ssd. It should be possible to make metacity-/marco load css themes instead of metacity ones.