Sure - create your css file and save it in your root directory where your web pages are stored. In this example it is
main.css
Then simply include it in the <head> section in the html file.
<link href="main.css" rel="stylesheet" type="text/css">
Code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Page Title</title>
<meta content="..." name="description">
<link href="main.css" rel="stylesheet" type="text/css">
</head>
