Help with .htaccess redirect coding

I am created a sub domain which works fine except that it has no styling. Having looked at several forums it seems like the browser needs to be directed towards the domain name for .css files. The style.css file is present in the subdomain but gives a 404 error when trying to find it using IE and Firefox.

At present the subdomain main page source contains:
“link rel=”stylesheet” type=”text/css” media=”all” href=”http://www.support.rdmac.org.uk/wp-content/themes/twentyten/style.css” /”.

I think it should be:
“link rel=”stylesheet” type=”text/css” media=”all” href=”http://www.rdmac.org.uk/wp-content/themes/twentyten/style.css” /”

I have created an .htaccess file in my subdomain folder and used the following code

RewriteEngine on
RewriteRule ^wp-content/themes/twentyten/style.css$ http://www.rdmac.org.uk/wp-content/themes/twentyten/style.css[R,L]

This is not working. Can anyone help?