How to link to your stylesheets in your theme
When you link to your stylesheets or javascript it is important that you use the <{$theme_url}> variable to get the correct address.
If you use slashes in the address on your products or landing pages etc. you will experience problems with loading a stylesheet if you just link via
<link rel="stylesheet" type="text/css" href="css/style.css" />
Because if you are on http://example.com/bluser/t-shirt your stylesheet will not be loaded and your page will not look correct.
That's why you need to use:
<link rel="stylesheet" type="text/css" href="<{$theme_url}>/css/style.css" />