Configure simplenews to include css in your newsletters

If you really want to change the way your newsletters appear in email client, you can do so using a custom mail.css file.

In Mail System settings, select the Theme used to render emails.

In that theme’s folder create a mail.css file.

Ex.

cd /sites/all/themes/adminimal_theme

vi mail.css

h1 {
        background-color: red;
        color : blue;
        font-size: 4em;
}

Refresh the cache.

Send the newsletter.

Note that the style is embedded in the email message.

Enable the Mime Mail CSS Compessor module.

Send the newsletter.

Note that now the css is applied directly to the element.

<h1 style="background-color:red; color:blue; font-size:4em">HTML Ipsum Presents</h1>

Which method you chose will depend on the results you get in the email clients your customers are using.

Note that hoping the sites css files will be used is a waste of time. Add a custom mail.css file and move on.