Insert an external CSS file in your HTML file like this. Remember to add this tag in the <head> section of the document:
<link rel='stylesheet' href='style.css' type='text/css' media='all'>
The media attribute is optional and it narrows down the scope of the stylesheet to certain devices. Here are the possible values: all, aural, braille, handheld, projection, print, screen, tty, tv.