While you can take it too far, in general try to break your formatting into smaller components. That way you can, for example, apply the heading styles to every page without also making them all blue.
It's common to simply name a site-wide style sheet main.css. But since you may eventually create multiple sites, make it a habit to include the site's name, such as mainTuffits.css, to avoid the confusion of having multiple main.css files.
Attach Style Sheets p. 65
In the CSS Styles tab, instead of right-clicking (Windows) or -clicking (Mac) the style rules, you also can use the buttons at the lower right: to attach a style sheet, to create a new rule, to edit a style sheet, and to delete anything.
Apply External Styles p. 67
If you think a particular style might be useful on many pages, work within the external style sheet. If, as in the example of centering Heading 3, you do not want to apply that particular style site-wide, use the Properties inspector to apply it to a single page.
Create Custom Rule p. 72
Officially when naming a custom rule in the New CSS Rule dialog box, you're supposed to start it with a period, for example .redItal. In practice, however, as long as you also select Class, Dreamweaver (Windows or Mac) automatically adds the period after you close the dialog box.
That same custom rule then can be used to apply red italic to any of your other styles.
-clicking (Mac) the style rules, you also can use the buttons at the lower right: 
to attach a style sheet, 
to create a new rule, 
to edit a style sheet, and 
to delete anything.