@ficbot - what you're looking to do is link the stylesheet you created to your HTML files.
In Sigil soon there will be a Link Stylesheets option for the stylesheets ( <-- note: Sigil related
) , but until then you have to manually edit your HTML code.
Add a link line to your head section like this (assuming your stylesheet is called styles.css but it could be anything.css):
Code:
<head>
<title>Introduction</title>
<link href="../Styles/styles.css" rel="stylesheet" type="text/css" />
</head>
Then save and reload your book just in case the information got cached and needs to be refreshed.