Customize almost everything in Thunderbird interface
Thunderbird is a powerful eMail application with a rather ugly interface. There exist some themes you can use, but it will be difficult to find a satisfactory one, especially since Thunderbird is updated continuously and the themes soon become useless. Here is a free and easy solution with impressive results.
You can change the background color in the folders and message headers pane, you can change fonts everywhere, etc.
Open Windows Explorer in the address of :\Users\YourAccount\AppData\Roaming\Thunderbird\Profiles\SomeName.default\chrome. You will find there the file userChrome.css, which controls the appearance of Thunderbird. Copy this file to a safe location in case you need it again, then open the original css file and the one you will download from this page, paste to the original all the settings you are interested in, and save it. Take care not to delete the top lines, those that start with a @, because these lines are needed by Thunderbird in order to work.
In the screenshot below you can see a possible color customization, but not the system font size (which is much bigger and easy to read).