The example you posted has the same icon for all the forums. I thought you were saying you want them different for each forum? If so, I think you would need to first edit the display_Forums_item template and use the nodeid to create a new class:
HTML Code:
class="forum-item {vb:raw forumCssPrefix} forum-item{vb:raw forumStatus} iconclass{vb:raw forum.nodeid}"
And then change the classes in the list to something like this (changeXX to the nodeid of the forum):
HTML Code:
.forum-list-container .forum-item.iconclassXX .cell-forum .icon {....