You can set up one external css file with all the style definitions for all the classes you can then use on any element of any HTML file.

Once you have defined styles in your css file for things like, for example, heading, menu, footer - then you can do things like

<div class="heading">A Heading</div>
<div class="footer"><The footer</div>

Now if you want to have different styles for different headings on different html pages, then you could create a style of heading1, heading2, etc and use a different class on a different page to make it look different.

Your first html page might use an element with class heading1 while your second html page might use the one called heading2. Then you could change the definition of either of those classes without affecting the other.