The name data-theme was a bit too generic, and implied that color

schemes and themes were the same thing. That boxes us in in the future,

and could clash with other uses.
This change renames it to data-ink-color-scheme, which is far more

clear on purpose, and can be reused amongst other themes in the future,

if ones were introduced.
It also adds "system" and "high-contrast" as options in Storybook,

Tested in Storybook and in Review Board with accompanying changes.