msglist: Area around mark-as-read button should be transparent (to `colorScheme.background`), not white

[chrisbobbe]

See this frame in the Figma: https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?type=design&node-id=147-9087&mode=design&t=BVHFiwepIX29vEdI-0

[chrisbobbe]

(ColorScheme.background is #f6f6f6, as we set it in 21dbae1.)