Thomas McMahon has knocked out some styles for the “Stylish” CSS-extension that produce brighter, better looking labels in Thunderbird.
The Stylish extension 
is a user style manager: “Stylish is to CSS what Greasemonkey is to JavaScript, and unlike other methods of using user styles, most styles take effect immediately.”
When you have installed Stylish, you can follow the instructions on Thomas’s web site to download some pre-made label styles that will turn your Thunderbird labels from this in to this:
And it’s not just a Mac-only solution as Thomas notes:
The new labels code has been tested in Thunderbird 1.5 and 2.0 on Mac and works great. It should work fine under Windows and Linux too.
Make Custom Thunderbird Labels Stand Out…
If you followed the Thunderbird Labels modification I put up a few days back, you may have found out that it didn’t work for custom labels/tags. Yes, I knew that, but wanted to start off with the basic labels. Here is a quick tutorial on how to…
I didn’t see this in a quick search, but it may have already been covered.
I was happy to discover yesterday that Thunderbird 2.0 has terrific support for IMAP keywords. There even seems to be a hidden feature. If you add a keyword and then “edit” it to rename it, the local display gets updated but not the remote keyword. This lets you put a happier face on an uglier keyword (e.g., keyword “Blah.Stuff.Goo.Label1″ gets displayed as “Work”).