Fortunately, we can fix this with an extension and some script.
First, go get the Stylish extension and install it.
Then go to this page where my script is: New Gmail – Visible Scroll Bar and click the Install with Stylish button. Once you do that and go to Gmail, the scroll bar will be a nice blue color. And it will stay blue no matter which theme you try.
If you don’t like my choice of blue, you can right-click on the Stylish icon at the top of your browser and then click Options. Then you click the Edit button under the name of the script. You’ll then see a line that says rgba(0, 122, 255, 0.7) The first 3 variables are RGB code (search around the web for help with finding the code to a color you like). The 0.7 part has to do with transparency so you can experiment with that. Just click save and go back to Gmail (or, easier, open Gmail in another tab).
Hope that helps some people out.