You'll want a link or import statement, like one or other of these two, in the head of your html document.
< link rel="stylesheet" type="text/css" href="yourstyle.css" />
Or, alternatively,
<style type="text/css">
@import url(yourstyle.css);
</style>
Of course you also need to have the css file uploaded and in the folder where the link refers to it. To preview locally in Safari from the file on your hard disc, you'll want to save a copy the css file into the same local folder / relative path where you saved the local copy of the html.
If you can't make this work, maybe post a link to where your page is.
