Display scroll bars in Webkit browsers

Scroll bars are currently hidden in Webkit browsers for the sidebar/right content panel since they were white by default. We should make sure they're visible and they respect the currently selected color scheme mode (light/dark).

Start by locating the sidebar and right content panel in the Apollo Client browser developer tools, then reproduce the hidden-scrollbar behavior in a Webkit browser with both light and dark color scheme modes. Done means scroll bars are visible in both panels and their appearance follows the selected mode.

Written by the indexing model from the issue text.