The CSS Editor allows you to enter custom CSS to control the appearance of a Campaign.
Visit a Campaign in the Campaign Builder and and open the Style Panel. Within the Style Panel, select CSS in the upper right corner.
The Custom CSS text area is where you can type or paste your CSS. It also has buttons to undo, redo, choose image, Validate, and Save the CSS.
The Validate button uses the W3 CSS Validator to check CSS for syntax errors. It’s important to click the Validate button after editing your CSS. It won’t help you with semantic problems, such as specifying green when you meant to specify red, but if you’ve forgotten a closing curly brace, the Validator will catch it. The Choose Image button to open the Image Selector. You can choose an image you've uploaded previously, or upload a new image. Once you have the image you'd like, click Select to close the Image Selector and past the image's URL into the CSS Editor.
As you type your custom CSS, the Campaign Preview will update to reflect your changes. This is especially helpful if you’re using absolute positioning.