By default, external cascading style sheets (CSS) open in CSS Editor. You can add styles either by typing style rules directly into the markup, or by clicking Build Style… and selecting options from the Modify Style dialog box, which automatically generates style rules and inserts their syntax into your document.
To open CSS Editor
On the Tools menu in Visual Web Developer, click Options.
The Options dialog box opens.
In the left pane of the Options dialog box, expand the Text Editor node, expand the CSS node, and then click Tabs.
If you have configured your environment settings for Web development, the Text Editor node is not available unless you select the Show all settings check box. For more information about environment settings, see How to: Change Select Settings.
Additional CSS options can be found under the HTML Designer node which is visible if the Show all settings box is selected.
For more information about style sheets, see Working with CSS Overview.