Install required styling (dev) dependencies

- babel-plugin-styled-components
- gatsby-plugin-styled-components
- polished
- styled-components
- styled-modern-normalize
- styled-theming

References:
  (1) https://www.npmjs.com/package/babel-plugin-styled-components
  (2) https://www.npmjs.com/package/gatsby-plugin-styled-components
  (3) https://www.npmjs.com/package/polished
  (4) https://www.npmjs.com/package/styled-components
  (5) https://www.npmjs.com/package/styled-modern-normalize
  (6) https://www.npmjs.com/package/styled-theming

GH-53

package.json

@@ -54,6 +54,7 @@
     "babel-eslint": "10.0.1",
     "babel-jest": "23.6.0",
     "babel-plugin-react-remove-properties": "0.2.5",
+    "babel-plugin-styled-components": "1.9.2",
     "babel-plugin-transform-react-remove-prop-types": "0.4.20",
     "babel-preset-gatsby": "0.1.4",
     "del-cli": "1.1.0",
@@ -85,14 +86,19 @@
     "gatsby-plugin-no-sourcemaps": "2.0.1",
     "gatsby-plugin-react-helmet": "3.0.2",
     "gatsby-plugin-remove-trailing-slashes": "2.0.5",
+    "gatsby-plugin-styled-components": "3.0.4",
     "gatsby-source-filesystem": "2.0.9",
     "gatsby-transformer-yaml": "2.1.5",
     "inter-ui": "3.0.0",
     "nord": ">=0.2.1 <1.0.0",
+    "polished": "2.3.1",
     "prop-types": "15.6.2",
     "react": "16.6.3",
     "react-dom": "16.6.3",
     "react-helmet": "5.2.0",
+    "styled-components": "4.1.2",
+    "styled-modern-normalize": ">=0.2.0 <1.0.0",
+    "styled-theming": "2.2.0",
     "typeface-rubik": "0.0.54",
     "typeface-source-code-pro": "0.0.54"
   }