iBooks Author: override
When you change a text attribute without applying a style, you create a style override. For example, if you select a paragraph that uses Body style and choose Format > Font > Bold, that paragraph’s style now has an override. When you select text that has a style override, the arrow next to the style name (in the Styles drawer) turns red. You can leave an override or do either of the following:
Remove an override: Click the red arrow and choose Revert to Defined Style from the pop-up menu.
Change the existing style so that it incorporates the override: Click the red arrow and choose Redefine Style from Selection from the pop-up menu. All instances of the former style are updated to the new style format.