Thank you for your post.
I believe you are referring to the Scroll Bar off to the right of the portal? There is currently no way to modify the scroll bar in FileMaker Pro or Pro Advanced. Certain themes (such as Electric) do have scroll bar customizations hard coded into the software, if that would be of interest to you.
Unfortunately, those aspects are not adjustable at this point. They are tied to the theme you're using. You could experiment with other themes.
Overlay a colored rectangle over the scroll bar and set the opacity to 50% for the rectangle.
I just reread your post and I don't think my answer solves the issue. Initially I thought you meant the whole scroll instead of just the slider.