Go to Google Images. Search "people of Walmart". Do NOT dress like what you see.
Seriously, your dress is whatever you feel would reflect well on yourself and our group. Personally, if I'm just in the gallery (not a directly involved party) my minimum would be *nice* jeans (no holes, not too faded, etc.) and a collared shirt (polo or button down).
As much, if not more, of importance is your manners in court, whether you are a party or a spectator. Be quiet. Do not react, positively or negatively, to what goes on in court. If you're a spectator, briefly cracking a *little* smile or frown to your friends is ok, but keep it to no more than that.
During the first hearing for CADL vs MOC, we had a big crowd of gun rights supporters, and there were a couple of instances of folks reacting audibly.
Let's keep our discipline in court and avoid that.