pk,
Cascading Style Sheets are designed to do just what you are describing. You create the external style sheet and apply it to all your site pages with a link command. Any changes you want to make to just one page can be done in the head section using style tags.
It would look something like this:
<head>
<title>your title</title>
<link rel="stylesheet" href="yourmainstyle.css" type="text/css">
<style type="text/css">
body {
background-image: url('/images/backgnd.gif');
}
</style>
You can also change anything you want different by styling it here inside the style tags.
For those single item changes, you can make inline changes, so maybe a paragraph needs to have a yellow background color and bold text, you could use this:
<p style="background-color:yellow; font-weight:bold;"> your text here </p>
For more information about CSS, you can visit the W3Schools website at
http://www.w3schools.com/css/css_reference.asp and even play around with some CSS on their site.