var ua = navigator.userAgent

var cssHTML = '<link rel="stylesheet" type="text/css" href="'

if( ua.indexOf("Mac") != -1 )        cssHTML += 'style_mac.css'

else if( ua.indexOf("Win") != -1 )  cssHTML += 'style_win.css'

else                                         cssHTML += 'style_other.css'

cssHTML += '">'

document.write(cssHTML)