There are two versions - one with the normal background and one with a background pic added just for fun and variety. The photo is a free one from Unsplash - you can easily change it in the stylesheet to any pic you would like to use for yourself.

To use, simply upload the stylesheet to your Nucleus/styles subfolder and rename it to admin_contemporary.css. If you're not already using the Contemporary design in your control panel you'll need to change that in the Configuration page as well :) If you don't see the difference, you may need to empty your cache first in case your browser is still using a cached copy of the original stylesheet.

If you don't like it, just reverse the process - delete or rename the modded stylesheet, and re-upload or rename the original stylesheet back to admin_contemporary.css. If you forgot to back up your original, just download a new copy of Nucleus from the NucleusCMS website and unzip it, you'll find the original version in the Nucleus/styles subfolder :)

If you are using one of the newer versions of Nucleus, you don't even need to rename the stylesheet. You can simply use the drop-down option selector in the Configuration section in the admin panel to choose the stylesheet you uploaded. It should show up automatically in the list once you have uploaded it to the right folder. (You may need to refresh or reload the Configuration page if you were already on it.)

The background pic should be uploaded to the same folder (Nucleus/styles). If you put it in a different folder, you'll need to edit the path to the pic in the stylesheet.

If you're wondering about the license, it just uses whatever the main NucleusCMS project uses, since this is based on the original stylesheets from there. Last I looked, I believe it was GPL v2.0.

Click on the screencap from my test blog above to see what the modified contemporary stylesheet looks like with a background photo added! (Some text has been blurred to protect the guilty ;)