If you build a lot of templates, you can use variables in your HTML and CSS to customize your email campaigns easier. You can then define the values (strings, colors, etc.) of these variables for each template on the Variables tab. For example, you can change the font or background color from one template to the next. For more information on defining variables, see Setting up email template variables.
As you add the value to each variable, you can see the result immediately in the preview pane.