Hey ! 
When I stumbled upon your post, I said to myself : “let’s see how it can be done”.
I found right away the CSS file wich define the style attributes of the chat, so I modified it, but none of the modifications that I did took effect.
I restarted, stopped, started the chat resource but nothing could be done to take my changes into account.
Then when I was looking aroud the server folders, I found out that their is a cache for the chat config !
So in order to do what you want to do :
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Modify the CSS file of the chat [ resources\ [gameplay]\chat\html\index.css ] (line 79) :

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Delete the chat cache wich is found in [ txData\CFXDefault_131746.base\cache\chat ] :

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Restart the chat resource :
/restart chat
Here is the result :
Peace !
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