This is the program scroll bar!
The thin scrollbars “on the folder list on the left and the message list on the right” are unfortunately currently restricted to thin scrollbars (even if you enable wide scrollbars in the Settings / Appearance / Lists).
As you can see below "only the message body has the wide scrollbar on the far right window using one of the default eg: Industry themes with wide scrollbars enabled.
This is due to the Chromium issue as @Michal_Burger advised in his post further up that should be resolved in eM Client 11. Hopefully that can be implemented in one of the first eM Client 11 builds.
