Dark theme has light scrollbars / scrollbars where none are necessary

Created by: beyang

• Issue type: bug report
• Sourcegraph version: sourcegraph.com
• OS Version: Linux, Chrome

Steps to reproduce:

1. Go to https://sourcegraph.com/github.com/Microsoft/TypeScript/-/blob/src/compiler/builderState.ts#L17:21
2. Set zoom to "90%" in Chrome (this is not necessary for all the scrollbars, but does cause a few more to show up.