This is the user style version of my Greasemonkey/Tampermonkey user script of the same name. They both accomplish the same thing. I made this for people who can't install user scripts but can install user styles.

More details and screenshots here on the corresponding script page:
https://greasyfork.org/en/scripts/1527-metafilter-recent-activity-border-o-rainbows

The screenshots here are mockup comments on the Classic theme; the style also works on the Modern, Modern Dark, and Plain themes.

Version 2.1 (* note: this is actually the first public release of the user style. The versioning starts at 2.1, to match the version of the user script on which it's based)

As of 2016-08-21: Tested to be working in Firefox and Chrome. Should work for all official MeFi themes (Modern, Modern Dark, Classic, Plain).

Credits

* This user script/style was inspired by a MetaTalk comment from phunniemee:
http://metatalk.metafilter.com/22732/A-Pony-with-a-Green-Check-Cutie-Mark#1094368

* The original rainbow cloud graphic is from a clip art library I got in the mid-1990s.