Glad to hear that you like the add-on!

I'd love to be able to do this. However, it's not currently possible due to the approach that Firefox takes when it comes to customizing the scrollbars. I really hope it will become possible in the future though because I think it would be really cool to give users like yourself more control over the appearance of the browser.

PS I left you a more detailed explanation on the request you posted on the GitHub repository: https://github.com/WesleyBranton/Custom-Scrollbar/issues/14