The Message Editor contains many native styling features that will cover the majority of your needs.
When you use a CSS, any settings you adjust in the message are taken into consideration based on the CSS settings. This means that styling defaults to the lowest level setting, whether that setting was defined in the CSS or in the message settings.
Copy the desired CSS from your document. To edit the message CSS, click on the Settings palette and then click Edit CSS.
Paste your CSS and click Done.
Be sure to use the following CSS classes to style your messages.
Be sure to use the full class; for example: .container-36469004145