Sorry to hear that the add-on wasn't working properly for you.

On some systems, the scrollbar colors may be overridden by the operating system itself. In that case, there is a setting that can be disabled in Firefox to allow the Custom Scrollbars add-on to change the colors. See this for more information: https://github.com/WesleyBranton/Custom-Scrollbar/wiki/Scrollbar-customization-is-not-being-applied