The styles included in the ribbon’s Styles and Markup Styles command drop-down selections, which appear in the Format Text tab when editing rich HTML fields, derive styles from the corev4.css file, located on the SharePoint Web front-end server at %SystemDrive%\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\1033\STYLES. By default, styles relating to the rich HTML field on publishing pages is prefixed with .
ms-rte. For example, the Comment selection in the Styles command drop-down appears in the CSS file as the style
By creating your own custom style sheets you can override default styles by using the same
.ms-rte prefix and create your own styles, ...