2) I selected the Sinistar skin on xmms. How did it achieve the transparency or non-rectangular windows? I believe there is a non-rectangular window X extension, is that the mechanism?
I don't know if this is "built-in", so to say. This shaped borders used to be around when i first tried enlightenment with Alien theme(1996). Back then, Enlghtenment relied heavily on Imlib, an image manipulation library. It's a combination of applying a maskcolor on your borderimages, and with some clever X11/Imlib tricks, one can achieve the desired effect.