A tad late, but here it is nonetheless: A collection of user styles that change the default rendering of websites. Perfect way to fool someone on April 1st.
Put these CSS definitions into your co-workers css overriding file. They will be applied to every website they visit. They are commented out by default, so make sure to uncomment your favourite ones!
Includes CSS to flip the entire site, flip all images, blur an entire site, etc.
Related: How to troll frontend web developers →
Related: Fool.js, a jQuery plugin containing some April Fools pranks to embed on your site.