as has been already stated, if the stylesheet on the PM website points to file:// uri, then it will be trying to get it from the server file system (instead of web server), which won't work. there are websites that do allow custom stylesheets, but that's old stuff anyway, as firefox and other browsers are bringing capability to override any website with your own stylesheet(s). which is what will work for PM etc. Just read up on your browser feature for overriding stylesheet..as it has nothing to do with setting a stylesheet on your PM settings
btw a good way to test any stylesheet is to set a 1px border for all elements. it makes all the boxmodels visible for all elements, and sure is visible.
border: 1px solid red
the hardest line to type correctly is: stty erase ^H