I'm assuming it's so you can have one specific icon for multiple forums right?
Maybe try something like this
[data-forumid="1"] .fa-comments:before,
[data-forumid="2"] .fa-comments:before,
[data-forumid="3"] .fa-comments:before {
content: "whatever";
}
idk if this'll actually work. But usually in css you can group classes together like that.