Is this a request, or are you wondering how to do it? :/
If you want to do it yourself, download WC3 Image Extractor (or open, if you already have it).
Then extract the files:
UI\Glues\Loading\Load-Generic\Generic-Loading-BotLeft.blp
UI\Glues\Loading\Load-Generic\Generic-Loading-BotRight.blp
UI\Glues\Loading\Load-Generic\Generic-Loading-TopLeft.blp
UI\Glues\Loading\Load-Generic\Generic-Loading-TopRight.blp
I think the rest is pretty obvious?
Those are 4 pieces that form the default loading screen, just change it to whatever you want and import the files with the exact same path in the world editor.
In this case, everything is black, apart from UI\...-TopRight.blp, which contains a little bit of text.
Set the loading screen to "Use Default Loading Screen" (if you used the campaign one).