This page uses a style sheet defined in a separate file. This allows changes to be made to many web pages at once by changing a common file. You can over-ride the general style sheet by rewriting the rules below the "link" tag. (Hence the name Cascading Style Sheets.)
This page was made to look this way by putting the following between the <head> and </head> tags:
<link type="text/css" rel="stylesheet" href="style.css">
<style type="text/css">
body { background: blue ; }
</style>
body { color: white ; background: black ; }