An CSS(cascading stylesheet)?

Cascading Style Sheets (CSS) is a style sheet language used to describe the definite design  of a document written in a markup language. Its most common application is to style web pages written in HTML and XHTML, but the language can also be applied to any kind of XML document, including SVG and XUL.
CSS is designed primarily to enable the separation of document content from document presentation, including elements such as the layout, colors, and fonts. This separation can improve content accessibility, provide more flexibility and control in the specification of presentation characteristics, enable multiple pages to share formatting, and reduce complexity and repetition in the structural content (such as by allowing for tableless web design). CSS can also allow the same markup page to be presented in different styles for different rendering methods, such as on-screen, in print, by voice (when read out by a speech-based browser or screen reader) and on Braille-based, tactile devices. While the author of a document typically links that document to a CSS style sheet, readers can use a different style sheet, perhaps one on their own computer, to override the one the author has specified.
Here an example when using an internal css:
<style>
h1 { color: white; background-color: orange !important; }
h2 { color: white; background-color: green !important; }
</style>
And an example for external css.
You use an seperate file for css. The using an extension .css its just a same style for css that include of class or ID open { attribute;semicolon } close thats what exactly creating a format file.