Add Theme UI

1. `yarn add gatsby-plugin-theme-ui theme-ui`
2. add `gatsby-plugin-theme-ui` to gatsby plugins array in `gatsby-config`
  `module.exports = {
    plugins: [
     'gatsby-plugin-theme-ui'`
     ]
    }
   `
   3. create a dir with an index.js in `src/gatsby-plugin-theme-ui/index.js`
   4. copy the basic colors object... there's more details in the docs
     `export default {
       colors: {
       text: '#333333',
       background: '#ffffff',
       primary: '#FF5BA3',
      }
    }`
    5. in your index import a button: import {Button} from 'theme-ui
    6. use yhe button somewhere` <Button>Boogy Time</Button>`