Please refrain from downgrading the chat resource to change its appearance, and rather change the CSS in the new one if you want to make it look different.
The old chat resource does not support various required features in FXS, and will cause issues in prolonged usage.
In addition, removing the chat resource to trigger the GTAO chat is not a supported scenario, as it will prevent resources from intercepting chat messages whatsoever, including commands.
To remove the box from the proper chat resource, open resources/[system]/chat/html/index.css and replace the following lines:
background-color: rgba(52, 73, 94, 0.7);
background-color: rgba(44, 62, 80, 1.0);
with a different color, alpha value, or remove them entirely.