Socialbase includes no grayscale/color styling for the "unread-thread" class

Problem

Colors are reserved for the socialblue theme which is good. However, the unread-thread class for the private message module contains no color changes at all in the socialbase theme which can lead to UX issues.

Suggested Solution

Let socialbase change:
- The background card color to be slightly off-white (from it's default, white).
- The read-indicator a background-color of something like rgba(0,0,0,0.2)

This introduces some grayscale values into socialbase to ensure that UX works on any theme that uses it while still allowing these values to be easily overwritten by subthemes (socialblue will continue to function as normal).