This extension allows the user to configure custom CSS that will be added to all email messages (including replies and forwards) when they are "composed".
Each time an email is opened in a compose window (write, reply, forward), the custom CSS will be added to a newly created <style> element in the email's head. If the head already contains a <style> element that was added by this extension, it is replaced.
For example, here is the configuration I use to make quoted text appear as expected, control the default font and width of the page, and set the font used for preformatted blocks:
blockquote {
background: #f9f9f9;
border-left: 3px solid #ccc;
margin: 1.5em 10px;
padding: 0.5em 10px;
}
body {
font-family: Arial, Helvetica, sans-serif;
max-width: 50em;
}
pre {
font-family: 'Courier New', monospace;
}
For more inforation on CSS, this is a good place to start:
https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Styling_basics
https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Styling_basics/Getting_started