"dark" style fixes

[helms-charity]

"dark" style should only be used to make all the text white.
Remove any background color, since "background-color" can be added as a separate section metadata item.

Pages where we see it being used on sections:
https://main--sling--da-pilot.aem.live/about
https://main--sling--da-pilot.aem.live/channels
https://main--sling--da-pilot.aem.live/freestream

I would remove top and bottom 8% margin from "dark" style. Instead, they can add the spacer on a per-use basis.