Thanks for pointing me in the right direction.
I have created a template called oscShop_headinclude, and put what is in the template headinclude into it, just to check that it working before I put my css in it.
In the Template: oscShop I changed {vb:raw headinclude} to {vb:raw oscShop_headinclude}
PHP Code:
{vb:stylevar htmldoctype}
<html xmlns="http://www.w3.org/1999/xhtml" dir="{vb:stylevar textdirection}" lang="{vb:stylevar languagecode}" id="vbulletin_html">
<head>
<title>{vb:raw vboptions.bbtitle} - {vb:raw pagetitle}</title>
{vb:raw oscShop_headinclude}
</head>
<body>
{vb:raw header}
{vb:raw navbar}
{vb:raw includeshopbody}
{vb:raw footer}
</body>
</html>
Can you tell me please what else I need to do.