DesignVariables.bgMessageRegular is slightly wrong in dark mode

[chrisbobbe]

I think the current value (HSLColor.fromAHSL(1, 0, 0, 0.11).toColor()) might have been taken from a value that web used in the past? Not sure how we ended up with a wrong value for something that's a named variable in the Figma.

Anyway, the Figma is clear: its bg-message-regular variable in dark mode is #1D1D1D.

Figma link: https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6262-90978&vars=1&var-id=3038-56382&var-set-id=2038-394378&m=dev and screenshot:

Successfully completing this issue requires:

• Changing the dark-mode value from const HSLColor.fromAHSL(1, 0, 0, 0.11).toColor() to const Color(0xff1d1d1d)
• Posting "before" and "after" screenshots in your PR description so we can see the change visually.