Based off an older Greasemonkey script, Gazelle at some point added built-in CSS classes for these, removing the need for JavaScript.

Note for snatched items: You may need to enable an option under settings.
Not all Gazelle-based sites use these CSS classes (most likely using an older version); this style will not affect those sites. I also cannot guarantee this will work/look perfect on every site, as I don't have access to them all.

2016-11-30:
* Removed now-defunct sites, added a couple more