📝(docs) add documentation about runtime theming

Add a documentation page about runtime theming.
It explains how to use the theming system
and provide a example.

Author: AntoLC

docs/theming.md

@@ -0,0 +1,33 @@
+# Runtime Theming 🎨
+
+### How to Use
+
+To use this feature, simply set the `FRONTEND_CSS_URL` environment variable to the URL of your custom CSS file. For example:
+
+```javascript
+FRONTEND_CSS_URL=http://anything/custom-style.css
+```
+
+Once you've set this variable, our application will load your custom CSS file and apply the styles to our frontend application.
+
+### Benefits
+
+This feature provides several benefits, including:
+
+*   **Easy customization** 🔄: With this feature, you can easily customize the look and feel of our application without requiring any code changes.
+*   **Flexibility** 🌈: You can use any CSS styles you like to create a custom theme that meets your needs.
+*   **Runtime theming** ⏱️: This feature allows you to change the theme of our application at runtime, without requiring a restart or recompilation.
+
+### Example Use Case
+
+Let's say you want to change the background color of our application to a custom color. You can create a custom CSS file with the following contents:
+
+```css
+body {
+  background-color: #3498db;
+}
+```
+
+Then, set the `FRONTEND_CSS_URL` environment variable to the URL of your custom CSS file. Once you've done this, our application will load your custom CSS file and apply the styles, changing the background color to the custom color you specified.
+
+