0.3.1-10 includes support for themed icons.
anyone can create a set of icons with the same filenames as the default theme and make them availiable to witter.
basically just create a folder under /opt/witter/icons/
that is the name of your theme, and in that folder place you icons (default icons are all 48x48)
then run witter and your theme will show up in preferences->select theme

themes could be made separately installable just need to put their files in the right place.
see my blog for full list of icon files required.