I already had a look at this related question, which covers changing the BFB icon in older versions of Unity. But the information there is probably dated, and does not entirely cover what I'm planning on doing in my theme.
I want to create an Ubuntu 15.10-compatible Unity 7 theme which mimics the current look of Unity 8 (used in Ubuntu Touch) as closely as possible. I'm now satisfied with how all the launcher buttons look, except for the BFB. Changing the icon inside the BFB itself should not pose a problem, but as you can see below the BFB background itself is differently-shaped from the other icons:
The Ubuntu Wiki's guide to theming Unity 7 explains how to create the background images for the launcher icons in general, but does not give any indication for whether it is possible to theme the BFB background separately. Does anyone know if this is possible to do in Unity 7, or is the BFB internal icon the only part that can be changed?