Default HTML font for new messages - Thunderbird

You can set a specific font, font color, and font size to be used as the default for all messages that you compose and send. To do so:

• Go to Tools > Options > Composition.
• Click on the HTML Options button and set the font, font size, and font color that you want. If you want to set a default background color to be used in messages you compose, you can also set that. Click OK.
• Click on the Send Options... button and make sure that the Text Format is set to anything except "Convert the message to plain text." Click OK, and then click OK again to exit out of the Options dialog.
• Next, go to Tools > Account Settings and select Composition & Addressing under the name of your account (in the left-hand pane). Make sure that "Compose messages in HTML format" is checked.
• If you have more than one account, repeat the previous step for each account that you want to use the specified font.
• Compose a test message to see if the above settings have taken effect. If not, then exit Thundebird and restart.
• When sending a message, if you are ever prompted about whether to send in plain text and/or HTML, make sure that you do not send in plain text only, since this will not preserve your font and background-color settings.