Improve themeGet helper

I made a suggestion in #368 that I think allows us to at least remove the fallback values from our styled-system themeGet() calls. Rather than this:

import {themeGet} from 'styled-system'
import {colors} from './theme'
const Component = styled('div')`
  color: ${themeGet('colors.green.5', colors.green[5])};
`

We can export a get() function from theme.js using styled-system’s own get() function to compute the fallback value:

import {get as getKey, themeGet} from 'styled-system'
export const get = key => themeGet(key, getKey(theme, key))

…which simplifies our components that use themeGet() nicely:

import {get} from './theme'
const Component = styled('div')`
  color: ${get('colors.green.5')};
`

This does allow us to get rid of defaultProps.theme in cases where we only use themeGet(), but other system props will still need theme to be set outside of a ThemeProvider to work properly. 😕

/cc @mxstbr

[emplums]

I think this could work! Regarding leaving theme out of defaultProps, we’ll still need that in every component so that the user has access to the theme for color and space props if a ThemeProvider is not provided

[emplums]

@shawnbot these solutions are cool, but I’m not sure how necessary they are IMO. Setting up system props, supplying a default, and setting the theme in one function feels a bit too much of a mixture of concerns for me and sort of what I was trying to get away from in the refactor. The defaultProps API is pretty standard and easy for people to jump in and know exactly what’s going on.