Dark Mode articles and design guides

Not an inverted stylesheet. This tag covers the parts that break when a light design gets a switch bolted on: type that goes muddy at the same weight, contrast that passes on paper and fails on an OLED at night, elevation that stops reading once shadows have nothing to fall on. Optical sizing matters more here than in light mode. Dashboards are the hard case, because they mix dense data with long sessions. We're specific about what to change and what to leave alone.