It would be truly ideal if we could change the color of a portal's scroll bar, which is currently defined & controlled by the theme.
Unfortunately, for many of the best themes, the portal scroll bar is very hard to see, and is almost indistinguishable from the portal itself.
For example, take a look at the screenshot below. This is a portal on the "Enlightened" theme. The scroll bar blends in so closely with the portal itself that it is practically invisible.
One of our clients is made up of older users who have difficult seeing the screen in the first place, but the nearly-invisible scroll bar is even more challenging for them. You can see that I've already added borders to the portal, but that only helps minimally.