Add util function to generate CSS Custom Properties from theme

[shanberg]

Instead of manually plugging in a variable for each item in the theme, we should automatically convert it to CSS variables. This also provides support for custom theme variables.

const ThemedCSSCustomProperties = (theme) => {
  return (Object.keys(theme).map(key => ('--' + key + ': ' + theme[key] + ';')));
}

export const LocalStyles = createGlobalStyle`
  :root {
    ${({ theme }) => ThemedCSSCustomProperties(theme)}
  };