Add support for extras_css option (#238)

source/plugins/core/README.md

@@ -55,6 +55,25 @@ Some templates may accept additional custom options that you can pass through th
     query: '{"custom_colo r":"#FF0000"}'
 ```
 
+### 🎨 Custom CSS styling
+
+You can inject CSS rules using `extras_css` option.
+
+If you make heavy use of this option, consider using [community templates](/source/templates/community/README.md) instead.
+
+#### ℹ️ Examples workflows
+
+```yaml
+- uses: lowlighter/metrics@latest
+  with:
+    # ... other options
+    base: header
+    extras_css: |
+      h2 {
+        color: red;
+      }
+```
+
 ### 🌐 Set timezone
 
 By default, dates are based on Greenwich meridian (GMT/UTC).