How might one go about designing new window borders etc for Gnome?

That's Metacity (the gnome window manager)... check here for Metacity theming instructions.

If you want to mod the controls like buttons and scrollbars, thats the GTK theme, not metacity - have a look here for GTK theming tips - they also have info about metacity and icon theming there too.

Hope that helps.

Thank you, looks like exactly what I needed.