yes!!,you can use external file for css and embedded it into html file.
There are three methods of including css in an HTML document: Inline styles — Using the style attribute in the HTML start tag. Embedded styles — Using the <style> element in the head section of a document. External style sheets — Using the <link> element, pointing to an external css file.
Tips:In good practice you should use external style because it can be reuse to other files as well.