Introducing two new colors:
- ColorTheme::color_dim, for dimmed/disabled primary colors
- SurfaceTheme::opaque_widget, for buttons.
In material design, a button's background color uses the Highest
Container role, which seems incorrect because then buttons wouldn't have
a different color when placed inside of the highest level container.
Rather than remove a container level, I added one more tone using the
neutral variant.
Other changes are just gut feelings to have a slightly richer dark
theme. I feel like material is a little muddy in dark mode.
