Ok, I guess this is a style request more than anything, but here goes.
For browsers that support full alpha pngs's.. It'd be nice if when you make a theme you could declare the opacity(sp?) and color of the drop shadow. right now i think the shadows are, by far, to opaque they stand out way too much. I think with a bit of coding, it would be easy to dynamically generate the dropshadow image.. or create it when you update your theme and store it for later use... What i'm getting at is a way you can define from the style how transparent you'd want your shadow to be and in what color. If I understand correctly, when you do this with an alpha png you only use 1 color and the full alpha channel, so this shouln't be hard to implement at all, also, for the vertical and horizontal gradients you wouldn't have to generate the whole image on the fly, just 1 row, and 1 column and then stretch them with HTML.. as for the corner, i guess thats a bit trickier, but this is just a suggestion, i'm sure you guys could figure it out, and if not, i'll try to think hard