How to add an alternative stylesheet to the browser

Add an alternative stylesheet to the browser

Set up a custom CSS file with some fun rules. (Tip: search for @twholman and fun.css for good ideas)
Add it to Safari by going to Safari > Settings > Advanced
Select your stylesheet under "Style sheet"

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If you want to find it easier or you want to prank someone

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Fun Mac pranks including rotating screen, swapping modifier keys, inverting colors, and customizing cursors