You can use CSS file with you html in 2 ways.
1. Embedding style tag withing html:
Look where is closing </head> tag. Then just before this tag insert style tag for example:
HTML:
</head>
<style type="text/css">
.styleOne {
font-family: arial;
font-size: 14px;
......
.....
}
#header{
width:940px; height:170px;
margin-left:auto; margin-right:auto;
background: yellow; color

urple; <--------write here color no. #999999
position:relative;
float:left;
</style>
</head>
now in the body tag u can use this div and class selector. for example:
<body>
<div id="header" class="styleOne">
<h1> This is a heading. </h1>
</body> ......................etc.
2. CSS file linking:
First create a style sheet file and save it with name "styles.css".
Then At the same place in html where i previous mentioned
just add this line of code
<link href="styles.css" rel="stylesheet" type="text/css" />
Note: make sure that this styles.css file is in the same folder where in html file.
Hoping this would be solved your problem.
Regards