Merge branch 'i/5696'

.stylelintrc

+{
+	"extends": "stylelint-config-ckeditor5"
+}

package.json

@@ -53,6 +53,7 @@
     "eslint-config-ckeditor5": "^2.0.0",
     "husky": "^1.3.1",
     "lint-staged": "^7.0.0",
+    "stylelint": "^11.1.1",
     "postcss-loader": "^3.0.0",
     "raw-loader": "^3.1.0",
     "style-loader": "^1.0.0",
@@ -75,11 +76,15 @@
   "scripts": {
     "build": "webpack --mode production",
     "lint": "eslint --quiet '**/*.js'",
+    "stylelint": "stylelint --quiet --allow-empty-input 'theme/**/*.css' 'docs/**/*.css'",
     "preversion": "npm run build; if [ -n \"$(git status src/ckeditor.js build/ --porcelain)\" ]; then git add -u src/ckeditor.js build/ && git commit -m 'Internal: Build.'; fi"
   },
   "lint-staged": {
     "**/*.js": [
       "eslint --quiet"
+    ],
+    "**/*.css": [
+      "stylelint --quiet --allow-empty-input"
     ]
   },
   "eslintIgnore": [