This diff adds styling so that unread subchannel messages are consistent with the rest of the web app, that is, they are made 'semi-bold' and that the color of the subchannel name, sender, text preview, and time are made white.
Deciding to change the color of all the subchannel elements was something I thought was appropriate but I'm happy to tinker some more with only color-changing certain elements.
This is one half of the linear task ENG-2089. The next step is to do this process for unread indicators for the sidebar modals.
