Add a css-lint workflow

.github/workflows/lint.yml

@@ -25,3 +25,34 @@ jobs:
         run: find . -name '*.php' -print0 | xargs -0 -r -n1 php -l
       - name: Lint all themes
         run: ./lint-theme.sh --all
+
+  css-check:
+    name: Check CSS files
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Set up Node
+        uses: actions/setup-node@v3
+        with:
+          node-version: 16
+
+      - name: Get Yarn cache directory path
+        id: yarn-cache-dir-path
+        run: echo "dir=$(yarn cache dir)" >> $GITHUB_OUTPUT
+
+      - name: Cache Yarn dependencies
+        uses: actions/cache@v3
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-
+
+      - name: Install modules
+        run: yarn global add postcss postcss-cli autoprefixer --non-interactive
+
+      - name: Run postcss
+        # postcss also checks for syntax errors
+        run: postcss */css/*.css setup/styles.css --use autoprefixer --replace