If you want to ensure that the appearance of your topics is consistent project-wide, one of the first things you should think about doing is creating a Cascading Style Sheet (CSS) and then assigning the CSS to all the topics in your project.
To assign the CSS to a single topic, open the topic. On the Formatting toolbar, click the Assign Style Sheet or Create New command. Select an existing CSS from the drop-down menu (or create a new one on the fly) and you are done.

Using the technique above will get the job done. However, you can only use this technique to assign a style sheet to one topic at a time. A typical RoboHelp project is likely to have hundreds (if not thousands) of topics.
On the Appearance tab, select the CSS you'd like to use from the list of Style Sheets at the left (in the Style Sheet area) and click the OK button.

Since multiple topics were selected before you opened the Topic Properties dialog box, all of the selected topics will now be using the same CSS.
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