If you mess up the CSS, you might be no longer be able to restore a working version, because the buttons are no longer visible.
Notes for security:
Open two browsers. Or at least two different browser windows.
In browser 2, open https://www.whonix.org/wiki/MediaWiki:Common.css [archive] as well. Make your edits in browser 2. See how they look like. And if something totally breaks, so you cannot fix it in browser 2 anymore, go back to browser 1 and just press "Save page".
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