When you change a style in a style sheet, all topics that are linked to the style sheet are updated to reflect the new formatting. (This is one of the major benefits of using style sheets instead of embedded or inline styles.)
From the Format menu, select Styles.
If the Hide inherited styles check box is selected, remove the check mark to see all styles in the Styles list.
Under Available in, select the name of the style sheet (*.CSS).
Under Styles, select the style you want to change (for example, Background+Text, Heading 1, or Normal).
Click Modify. The Modify Style dialog opens.
Under Style for following paragraph, select the style you want to apply to subsequent paragraphs from the drop-down list.
Click the Format button, and select any of the following options to change the style:
Font: Selects font families, size, and attributes (bold, italics, etc.).
Paragraph: Sets up indents, line spacing, and alignment.
Borders and Shading: Borders tab sets border types, color, line thickness, and spacing. Shading tab assigns background color and image options.
DHTML Effects: Assigns the type of effect to include and when to include it.
Bullets and Numbering: Sets bullets and numbering styles.
When you finish modifying the style, click OK.
Repeat steps 4 through 8 for each style you want to change.
Click OK when you finish. The changes are saved to the style sheet. All topics that are linked to the style sheet are updated to reflect the new formats.
Note: Any changes you make to an external style sheet are incorporated into every topic attached to the style sheet. Once your changes have been saved, they become part of the style sheet. You cannot undo applied changes to an external style sheet.