A style guide is a collection of settings that generically determine the look and feel of all (or a large subset of all) windows/dialogs. All these windows/dialogs will inherit this look and feel unless different specifications are made for them locally.
The style guide of the Windows Designer works much in the same manner as the style (or template) mechanism used in professional word processors. Just as you would develop different style sheets for the various types of paper documents (letters, memos, invoices etc.), so you develop Windows Designer style sheets by setting properties for the various types of windows, dialogs and controls.
Two features help you design a style guide:
An application's style guide concerns GUI look and feel WITHIN that application. If you want to use the same generic GUI style in several applications, or to re-use one application's style guide in another, use GUI applications as templates.