Fix the color of the centered comment FAB in dark mode

[micahmo]

Pull Request Description

This PR adds a background color to the centered post page FAB in dark mode. I chose not to add this color in light mode because I think the FAB already stands out well against the comments due to the shadow.

I tested...

• the old and new post pages
• light and dark mode
• combined and separated FAB

Issue Being Fixed

Issue Number: #1689

Screenshots / Recordings

qemu-system-x86_64_wspLkWkSp6.mp4

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

LGTM - I do personally prefer a more muted colour, but we can roll with this for now and get some additional feedback!

[micahmo]

Agreed!