Fix "theme" taxonomy

[mattkrick]

Surfin through the stuff I noticed theme can mean different things in different places & I wanted to get to a place where we can all agree on the same words.

So, I propose:

• appTheme: The thing that comes from theme.json (We should probably rename that to match). contains brand, palette, typography. The object is completely static. Servers as a defaultTheme when the desired value does not exist in customTheme below.
• colorPalette: a string like 'cool', 'warm', 'dark' that is passed into a generic component like Button or IconLink. Runtime.
• customTheme: The team-specific colors passed in via context. Currently not implemented. At runtime, the customTheme is deeply merged with the defaultTheme & the output is supplied to a styleThunk (the thing that creates the styles object at the bottom of a component).

With these 3 things, we'll be pretty unstoppable. I imagine something like this:

const styleThunk = (customTheme) => ({
  root: {
    color: customTheme.primary // defaults to appTheme.palette.primary
  }
})
export default withStyles(styleThunk)(Button)