In Dreamweaver CC and later, CSS Styles panel is replaced with CSS Designer. For more information, see CSS Designer.
Design-Time style sheets allow you to show or hide design applied by a CSS style sheet as you work in a Dreamweaver document. For example, you can use this option to include or exclude the effect of a Macintosh-only or a Windows-only style sheet as you design a page.
Design-Time style sheets only apply while you are working in the document; when the page is displayed in a browser window, only the styles that are actually attached to or embedded in the document appear in a browser.
You can also enable or disable styles for an entire page using the Style Rendering toolbar. To display the toolbar, select View > Toolbars > Style Rendering. The Toggle Displaying of CSS Styles button (the right-most button) works independently of the other media buttons on the toolbar.
To display a CSS style sheet at design-time, click the Plus (+) button above Show Only At Design Time, then in the Select a Style Sheet dialog box, browse to the CSS style sheet you want to show.
To hide a CSS style sheet, click the Plus (+) button above Hide At Design-Time, then in the Select a Style Sheet dialog box, browse to the CSS style sheet you want to show.
To remove a style sheet from either list, click the style sheet you want to remove, then click the appropriate Minus (–) button.