Hello,
You cannot set a favicon by using CSS.
It can only be done like so:<link rel="icon" href="/images/favicon.png" type="image/png" />
If your KMS is self hosted on your own ENV, you can just override the default under $KMS_DIR/public/favicon.ico.
If it is hosted on our SaaS, there is a custom module called customfavicon which you can ask to enable by contacting your Kaltura account manager.