edit scroll bar color in Tvertscrollbox fmx

Hello, Using FMX, Tokyo 10.2, target OSX. I'm trying to change the color of the scroll bar in a Tvertscrollbox.

When I click on Edit Custom Style, or Edit Default Style, while the component is selected, I get an error message "Cannot find style resource for ."

Am I doing something wrong, or is this a bug?

And either way, is there a better way to change the color?  The form has a dark background so the black scroll bar is hard to see.

Thanks.

Scott