How to load a different css file from a subdomain with the same database?

I working my self in to the mobil phone redirect to subdomain world. And in my case (so far) it’s all about changing the css file when entering m.mydomain.com from a cellphone (or being automatically redirected there via http://blog.mobileesp.com/, using the same database and content as from mydomain.com.

So, how is the easiest way to load the mobile.css from m.mydomain.com? (I’m not talking about the <link media="handheld" /> in this case.)

Thanks.