refact: extract index.sass to styled-components

package.json

@@ -17,7 +17,6 @@
     "gatsby-plugin-prefetch-google-fonts": "^1.4.2",
     "gatsby-plugin-react-helmet": "^3.1.2",
     "gatsby-plugin-react-svg": "^2.1.1",
-    "gatsby-plugin-sass": "^2.1.3",
     "gatsby-plugin-sharp": "^2.2.9",
     "gatsby-plugin-styled-components": "^3.1.2",
     "gatsby-plugin-typescript": "^2.1.2",
@@ -27,7 +26,6 @@
     "gatsby-transformer-remark": "^2.6.9",
     "gatsby-transformer-sharp": "^2.2.4",
     "highlight.js": "^9.15.8",
-    "node-sass": "^4.12.0",
     "prismjs": "^1.17.1",
     "ramda": "^0.26.1",
     "react": "^16.8.6",