visto che non avete saputo aiutarmi (non per colpa vostra naturalmente) mi rispondo da solo così se a qualcuno interessa...

if ( navigator.userAgent.indexOf("Mac") != -1 )
{
if ( ( navigator.appName.indexOf("Netscape") != -1 ) && ( parseFloat(navigator.appVersion) <= 4.7 ) )
document.write("<link rel='stylesheet' href='stili_mac.css' type='text/css'>");
if ( ( navigator.appName.indexOf("Netscape") != -1 ) && ( parseFloat(navigator.appVersion) > 4.7 ) )
document.write("<link rel='stylesheet' href='stili_mac.css' type='text/css'>");
if ( navigator.appVersion.indexOf("Explorer") != -1 )
document.write("<link rel='stylesheet' href='stili_mac.css' type='text/css'>");
if ( navigator.appVersion.indexOf("Safari") != -1 )
document.write("<link rel='stylesheet' href='stili_mac.css' type='text/css'>");
if ( ( navigator.appName.indexOf("Netscape") == -1 ) && ( navigator.appVersion.indexOf("Explorer") == -1 ) && ( navigator.appVersion.indexOf("Safari") == -1 ) )
document.write("<link rel='stylesheet' href='stili_mac.css' type='text/css'>");
}
if ( navigator.userAgent.indexOf("Mac") == -1 )
{
if ( ( navigator.appName.indexOf("Netscape") != -1 ) && ( parseFloat(navigator.appVersion) <= 4.7 ) )
document.write("<link rel='stylesheet' href='stili_win.css' type='text/css'>");
if ( ( navigator.appName.indexOf("Netscape") != -1 ) && ( parseFloat(navigator.appVersion) > 4.7 ) )
document.write("<link rel='stylesheet' href='stili_win.css' type='text/css'>");
if ( navigator.appName.indexOf("Explorer") != -1 )
document.write("<link rel='stylesheet' href='stili_win.css' type='text/css'>");
if ( ( navigator.appName.indexOf("Netscape") == -1 ) && ( navigator.appVersion.indexOf("Explorer") == -1 ) )
document.write("<link rel='stylesheet' href='stili_win.css' type='text/css'>");
}