I would run a couple sanity checks. I can think of two immediate things which could affect the scenario you are describing.
1) Make sure the harness and CSS [selector] applies to the scenario you are trying to test. Do something you know and can easily identify like changing the background color.
2) If the above suggestion still does not work, apply an !important tag temporarily and see if that works. If it does, then your CSS is being applied but something else is taking precedence due to the inheritance hierarchy.
Based on these tests use the F12 dev tools to try and identify what might be taking precedence over your changes and/or why your CSS is not taking effect.