Tweak margin around chat system messages

[JohnHKoh]

Changed margin-bottom: 10px; to a margin: 5px; to add a little more space around system chat messages. I didn't like how close the message was to the horizontal bar below the player names. No one really asked for this, but I think it looks nicer 🤠 If there are more system messages to be added in the future, this will come in handy.

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Name	Status	Preview	Updated
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[stevenhao]

lgtm!