Quinnos, create a new subfolder in your \UI folder and place all your custom files in there. When the game loads, it will use your custom files first instead of the default versions. To go back to the default for any file you have customized, just remove it from the custom folder.
To make the game use your custom UI folder, log in and type
/loadui Custom
where
Custom is the new folder name. You only have to do this once to set it as your preferred UI folder.
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