If you've added the style sheet to the ePub using Sigil, then it HAS
to be in the "Styles" folder. That's where Sigil puts any file you add that has the .css extension. So make sure it's in the "Styles" folder (when in Sigil)... and if it's named "style.css", then the link code will be:
<link href="../Styles/style.css" rel="stylesheet" type="text/css" />
The folder you have the ePub saved in on your hard-drive isn't relevant. Only the paths within
the ePub are relevant.