Are you trying to change the margins for when you Print an email?

'File' > 'Page Setup' is used to alter the page margins when you need to print the email. It is not used to create a Template email nor to change the overall theme used in the Thunderbird user interface. To add a theme to the overall Thunderbird window, you can choose * either a Windows theme via Control Panel > Personalization >Theme * or download a theme Add-on, there is a huge range here: https://addons.mozilla.org/en-US/thunderbird/themes/ such as : https://addons.mozilla.org/en-US/thunderbird/addon/balloons/?src=ss If you are looking to create a Template email, then you open a new Write message, design the email, enter a Subject so that you can easily locate the email and click on 'File > Save as > Template or click on Save button and choose 'Template' from the options. then the email is saved in the Templates folder. To use, you click on Templates folder and double click on the email to open in a new Write message. then compose and send as normal. The original email will be saved in Templates folder for reuse.