Surface and background colors don't match spec

[anthonymoretti]

In scheme.dart the tone values don't match the spec.

Expected

background: palette.neutral.get(98),
surface: palette.neutral.get(98),

background: palette.neutral.get(6),
surface: palette.neutral.get(6),

Actual

background: palette.neutral.get(99),
surface: palette.neutral.get(99),

background: palette.neutral.get(10),
surface: palette.neutral.get(10),

[MrEbbinghaus]

Reference: #114 (comment)