Pidgin is heart-breakingly close, but as far as I can tell, it's not there yet:
- Pidgin-WebKit would be perfect, except it doesn't seem to compile on Windows.
adium2pidgin-themes converts Adium Xtras into Pidgin themes, but only supports sound, status, and emoticon theme types:
-t TYPE, --type=TYPE type of theme, may be: auto, sound, status or emoticon, default: auto
The Pidgin project is considering merging Pidgin-WebKit into Pidgin itself, but that sounds like a long way off:
Most notably, we've been talking about merging the webkit integration branch into what will become 3.0.0. Eventually, this would allow the support of Adium's message styles, although it may not happen right away.
So, are there any Windows IM clients that support Adium styles today?