[Belthan]

You are correct about the color of the hand icon. If you pick up something while it's red you get credit with the TG for stealing, you get a bounty if you get caught, and you can only sell it to a fence - honest merchants won't buy it. If you pick up something while it's white, it's fair game - no bounty even if somebody sees you do it, and it won't apply to your independent thievery goals.

I would say you are certainly not a terrible thief if you can get past Baenlin's bodyguard. You'd just be better off picking a different house to burgle. The most likely reason that so many things in Baenlin's house don't count as stealing is that Baenlin and his house are involved in another quest (SPOILER: it's one of the DB quests, and being able to sneak past Baenlin's bodyguard will come in handy in that one too).

edit - p.s. be careful stealing in Bruma because several TG members live there and you'll get kicked out of the guild for stealing from them, plus over half of the houses are involved in some way with other quests that might be harder to complete if the owner doesn't like you because he or she caught you stealing. Saving before you burgle any home in Bruma is probably a reasonable precaution.