Add this to the very bottom of StackOverflow's style sheet (or use stylish to test it)
It instantly will soften all the hard edges for almost all the boxes on your site that drive my eyes crazy. The tags, etc. instantly become softer.
Unfortunately there are still a few items that don't have borders so it won't work on them (like the horizontal menu at the top of each page) but it's still a great improvement. This is subtle, but it makes a difference.