Use the Change Stylesheet dialog to assign a stylesheet to the document.
Use the New Stylesheet dialog to create a new stylesheet.
The dialogs will be described below:
1. Change Stylesheet
Use this dialog to assign a stylesheet to the document.
It can be opened by pressing the Change Stylesheet button in the Styles group under the Home tab and selecting Assign Stylesheet from the list.

In this dialog box type the path of the .css file that contains the styles or click Browse and look for it in computer then click OK.
When you assign a stylesheet to the document, a list with the available styles will be enabled in the left side of the Styles group.
To assign a style to an element from your document:
- Assign a stylesheet to the document.
- Select the element or go to the Navigation Bar and click on the item for which you want to apply the style.
- Click on the bottom arrow near the list with available styles in the Styles group.
- Select the desired style by scrolling in the list.
2. New Stylesheet
Use this dialog to create a new stylesheet.
It can be opened from the Styles group under Home tab. Press the Change Styles button and select New Stylesheet from the list.

In this dialog enter the path and the name of the new .css file or browse for it in the computer.
Click OK and the Manage Styles dialog will open where you can add new styles to your file by pressing the New Style button. The styles can be modified after creating them by using the Modify button.
See also:
How to apply styles to a document