CSS stands for Cascading Style Sheets, is the recommended way to control the presentation layer in a web document. CSS covers fonts, colors, margins, padding, height, width, background color/ images etc.
<style> body{ background-color: #FF0000; } </style>
You can write the css code in-side your HTML pages or you can include external css files. It is always recommended to use external css files rather writing the css in individual HTML pages.
<link rel="stylesheet" type="text/css" href="path2yourfile.css" />