Lua has no access to reading the chat window so this is not currently possible.
What I do for this is to create a new chat tab, filter it to only Standard, then change the font color and size for Standard chat and finally, pull the tab out of the chat box and put it above the regular chat. Unfortunately there is no way to turn off the loot spew that goes there so it kind of sux when someone is looting NPC corpses while you're busy fighting, but it still works fairly well.
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