I don't use the Shoutbox on a regular basis, but I did find this rule:
I bolded the parts that I think apply here. I understand that it seems kind of an over-application of the rule in the case of helping someone, but the rule is there probably for a very good reason.
If you want to point out a forum thread to someone you were talking to in the shoutbox, you can always comment on their profile or send them a smail with the relevant information. :) In some cases, it may even be appropriate to @ them on the right thread that you think they may be interested in. I don't know about in the CSS threads, but in the Adoption Center I see people @ other users all the time for pets they think they might be interested in.
ETA: If you have a question about the rules, it's always helpful to file a ticket about it. Admin can always provide better clarification than us users can. :)