This extension will add a different style to ACP.
Background colour can be changed in ext\dmzx\acpstyle\styles\all\theme\acpstyle.html to whatever you like.
Unzip the downloaded release, and change the name of the folder to `acpstyle`.
In the `ext` directory of your phpBB board, create a new directory named `dmzx` (if it does not already exist).
Copy the `acpstyle` folder to `/ext/dmzx/` if done correctly, you'll have the main extension class at (your forum root)/ext/dmzx/acpstyle/composer.json
Navigate in the ACP to `Customise -> Manage extensions`
Look for `ACP Style` under the Disabled Extensions list, and click its `Enable` link.