If you want the entire look of Thunderbird to be using a light background then change the theme to 'Light'. Menu app icon > Addons Select 'Themes' Look for 'Light' theme and click on 'Enable'

If you want to keep the 'Dark' theme but have emails in the 'Message Pane' to use a White background instead of a dark background.

Menu app icon > Options Select 'General' Scroll to 'Language & Appearance' section click on 'Colours' button Set 'Text' colour to Black (or blue whater you prefer) - click on the coloured rectangle to choose a colour Set 'Background' colour to white

uncheck 'Use System colours' Override the colours specified by the content with my selections above: Set as 'Never' to allow html css to work in recived emails. click on OK

To set up default Text and Background colour for emails you want to send: Menu app icon > Options select 'Composition' Under 'HTML Style' section

To use same Text and Background as set up in 'Language & Appearance' - see previous comment. Select 'Use readers default colours' Otherwise uncheck that option and choose the Text and Background' colours.

Once default is set up in Options, when composing in HTML, you can also change the Text or Background colour on a per email basis by using the 'Formatting Bar'.