To change the look of Thunderbird , you can make use of the different themes available on https://addons.mozilla.org/en-US/thunderbird/themes/
- Simply download the theme of your choice
- Open Thunderbird, click on Tools > Add-ons > Themes
- Click on the Install button
- Browse for the downloaded theme (.xpi)
- Click on Open
- In the window that opens click on "Install Now"
- Once the theme is installed click on the "Restart Thunderbird" button to apply the new changes.