Yes... and yes.
The stylesheet goes in the Styles folder. And you link each xhtml file (that you want to use it) to that stylesheet with:
Code:
<link href="../Styles/stylesheet-name.css" rel="stylesheet" type="text/css" />
in the head section.
If you have a lot of xhtml files, it's fairly painless to pop the <link /> tag in with Sigil's Find and Replace utility. If you don't have any stylesheets already, just find </head> and replace it with:
Code:
<link href="../Styles/stylesheet-name.css" rel="stylesheet" type="text/css" /></head>
"All HTML Files" or "Selected HTML files"