At the moment creating a new png file for a theme requires changing an existing single layered image with a transparency. Changing the colour requires mixing it with another colour or a repaint. The former can affect the quality of the colour and the the definition of the icon on a button. The latter is more time intensive and requires skill getting the icon right, or loosing the texture.
If we had a set of vanilla layered files to work from . With the icon on one layer, the button texture on another, a fill colour for the button, and a transparent layer. Changing the theme's would become much easier.
It's been suggested that their is a lighter them in the pipeline, but I think this would enable us to create a variety of quality themes much faster, so can we have this instead, or in priority of.