Joe, I copied the exact CSS from your linked help file to a file, search/replaced the file names to match my actual file names on the disk and … when imported, there are no errors, but it does not work. The player remains the default player, totally ignoring the custom skin. This is an actual bug.
Honestly, if you want to allow customizing the player and the reaction to some trivial error is to ignore the file entirely, you need to create a GUI skin editor instead of expecting users to debug undocumented text config files for you. I’m a Unix systems admin (and documentation writer) and I find this to be difficult. (It doesn’t help that the MediaElement.js documentation is atrocious.)