2016-08-21: Now also available as a user style for the Stylus extension (I plan to move this to mefiscripts github): https://userstyles.org/styles/131769/metafilter-recent-activity-border-o-rainbows
[This script was originally hosted at userscripts.org]
Quick note: This script works fine with the MeFi Modern Theme. Will try to add screenshots when I next have a chance.
Example with the border o' rainbows script in action [this and the following image are mockups, not actual comments]:

Example without the border o' rainbows script:

Change log:
- Version 3.0: 2020-01-11 - Replaced GM_addStyle (deprecated in Greasemonkey 4.0). (Thanks to Tehhund for reporting the issue) Tampermonkey should still work with version 2.1 of this userscript.
- Version 2.1: 2014-05-23 - Replaced @namespace with script home page.
- Version 2.0: 2014-05-23 - Changed script URLs to point to greasyfork.org since userscripts.org hasn't been stable recently.
- Version 1.1: 2013-11-01 - Added support for automatic script updates (had to upload to userscripts.org first to get the URLs).
- Version 1.0: 2013-11-01 - Grand opening.
What this script does:
If you're logged into MetaFilter and you've authored any comments that are recent enough to appear in the "Recent Activity" section, normally your comments have a solid border around them. This script replaces that line border with lots of little rainbows (atop puffy clouds, no less). Works for both the "Recent Activity on My Posts & Comments" and/or "Recent Activity on My Favorite Posts" pages.
This script was inspired by a MetaTalk comment from phunniemee. I didn't have a chance to work on it then, but since no one else has posted anything similar yet, here it is.
(Here's some further info from pb about the border around a member's own comments in Recent Activity.)
Like the rainbows? Consider donating to help support MetaFilter.
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This script works with the MeFi Modern, Classic, and Plain themes.
Browser compatibility: Version 3.0 has been tested as still working in Firefox 72.0.1/Linux Mint. It should work in Tampermonkey/Chrome as well. No guarantees that this script will work anywhere else or with other MeFi-related scripts (e.g. it has not been tested in Opera).
If you'd like to modify the script, please feel free. Disclaimer: I am not a programmer, but if you have suggestions for improving it, I'd be interested in learning about them.
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Credits:
The original rainbow cloud graphic is from an Art Explosion clip art library I got over 15 years ago. (I tried to find other pony/rainbow/heart images (re: phunniemee's original comment) that would work okay in a border context while scaled down small without being too distracting; ended up going with this simple graphic to create the border.)
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More rainbows/animals/cuteness for your MeFi experience:
If you like this script, you might also want to check out these other scripts I posted: an update to Rhomboid's MetaFilter unicorn and narwhal buttons and laser kitty (with more rainbows), MetaFilter hedgehog comment pointer, and MetaFilter inline video fishy icon.