CSS:
.node.node--idX {
&.node--forum {
.node-icon i:before {
content: url(path_to_your_image);
width: 32px;
}
&.node--read .node-icon i:before {
opacity: 0.5;
}
}
}
With this code opacity will be applied to image when forum is read, and image will be without opacity when forum is unread.
it will works for basics forums
So i think for search forum just add
.node-search
after
&.node-forum
@BassMan style