The only problem with this phrase is that while Thunderbird is vastly customizable most people are unaware of the basics of the process. The support forums have seen many an unhappy camper since the Aero Theme was introduced as default, but it is only one Theme and Thunderbirds appearance is customizable with hundreds, if not Thousands of Themes.
The following are four Thunderbird themes which can simply replace the appearance Mozilla gives to Thunderbird
The best approach to get a theme is to use the add-on manager on the tools menu. When it opens the tab should by default show the get add-ons tag. If it does not click on the get add-ons. By searching through the Themes this way you get to simply click the install button. If you do it using your browser, you have to download the files and manually install it. You also get the benefit of only seeing Themes that are compatible with your version of Thunderbird. This can save time if you are one the current version, but can cause a few problems occasionally if the Thunderbird version is not up to date. Sometime it limits results a little to aggressively.
Before you ask for new functionality, check out the add-ons site. it is the official repository of addons for Thunderbird. This site contains hundreds nay Thousands of addons most of which add useful functionality of some sort to Thunderbird. Simply because there are so many add-ons, the site contains collections. These are groups of add-ons that users have collected together as they feel they make a set. I have a collection of the add-ons that get refered to often on the support site at Get Satisfaction. If you are looking for a place to start, there is a chance I have included it in my collection.
One thing to be sure about is that the addon site is not the only place you can get addons. One place I constantly recommend is Kaosmos website This is the home page of some of the best add-ons for Thunderbird. In particular, have a look at the quote and compose manager. A must have for everyone as it has an option to stabilize the font when composing a message. Without it the font you have selected keeps changing as you move around in the message you are writing.
Remember, that add-ons at the official site are reviewed. Keasmos also is also a site that can be trusted but it is possible that an add-on can be harmful, so be sure when installing add-ons from sites other than these two that you read what other people have to say about the add-on. Also be particularly careful of add-ons installed by anti virus and SPAM software. While these are not intentionally harmful, they often break other functionality in Thunderbird.