I found it in 920style (Conversation style) not in style 1001.
If you want modify the background:
code:
convhistory[class="DefaultWallpaper"]
{
background:rcbkd(59xxx);
}
convhistory[class="CustomWallpaper"]
{
background:rcbkd(59xxx);
}
convhistory[class="DynamicWallpaper"]
{
background:rcbkd(59xxx);
}