CSS Media Queries are also well and good after you realise that the problem you’ve been trying to fix comes from being too particular and drilling down to specify paragraph formatting inside the div, but the replacement stylesheet only overrides the container div, not the paragraph formatting.
Urgh. At least I’ve got the problem fixed.
Also, adding .content:after{content:"Mobile Stylesheet";} is a good way to see if the stylesheet is actually being loaded during testing.