Pro FeaturesYes. Highlight some text in the composer and use the formatting toolbar below the subject line to change the font, text size, and color, along with bold, italic, lists, and more.
See Formatting messages for the full list of formatting options.
Making everything bigger (or smaller)
To change the size of text in the message list and in emails you read, use the Scaling slider in Preferences > Appearance. It scales the whole interface — text, icons, and spacing — without changing the messages you send. If wide emails are hard to read, also try Restrict width of messages to maximize readability in Preferences > General > Reading.
There's no setting for a default composer font yet: new messages use the theme's font and the recipient's mail client decides how to render them. Pick a font from the toolbar for individual messages, and stick to widely available fonts (Arial, Georgia, Verdana...) so they look the same for your recipients.