Hi Squillace =)
You just need to add new background ids to the windows you want to change.
Conversation is 920, if I recall correctly, and don't remember that popup right now, just look for it.
Do this: (being ~ the path to your Messenger Plus! Skins folder)
Go to ~/My Live Messenger/Windows/Definitions and open the definition you want to change (msgres_def_920.txt for 920 for example), and look for "background=rcbkd("msgr",625)" (which is the the "main" background).
Just change it to any number between 59990 and 59999 (I'm sure I'll never use those, so there won't be problems), and declare that new number in SkinInfo.xml, indicating the correct path to the background and everything.
If you want things faster, simply replace a background file keeping the original name over custom's name, and only declare that new number in the corresponding ResGroup (use Ctrl+F with the file name).
Anydoubts, I'm here