This is a difficult question to answer. Likely, the css is being loaded just fine, but the content fails to successfully select any element to apply its style to.
To test that the css is being loaded, put a unique string in the file as a comment (e.g. /* You can't find this in any other file. */). Then open the console (ctrl + shift+I) and search in the element windows for your string. It should appear in one of the loaded styles.
In the element window of the console, if you can select the elements you expected to style and expand the applied styles to see if yours is being overridden by a more specific selector.