Add postcss

Author: tolyo

docs/package.json

@@ -38,6 +38,7 @@
     "autoprefixer": "^10.4.21",
     "cross-env": "^7.0.3",
     "hugo-extended": "0.147.8",
+    "postcss": "^8.4.38",
     "postcss-cli": "^11.0.1",
     "rtlcss": "^4.3.0"
   },