Update theme guide for Chamilo 2.0 with new structure and instructions

THEME_GUIDE.md

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+# Theme Guide for Chamilo 2.0
+
+This guide explains how to create and customize your own theme for **Chamilo 2.0**.
+
+## Folder Structure
+
+In Chamilo 2.0, themes are stored in the new directory layout:
+
+1. `public/themes/{your_theme}/`
+2. `resources/views/themes/{your_theme}/`
+
+> Replace `{your_theme}` with your theme’s name, in lowercase.
+
+## Files Overview
+- **CSS/SCSS files** go inside `public/themes/{your_theme}/css/`
+- **Images** go inside `public/themes/{your_theme}/images/`
+- **Twig templates** (HTML structure) go inside `resources/views/themes/{your_theme}/`
+
+## Naming Rules
+- File names should be **lowercase** and use `.css` or `.scss` extensions.
+- Example: `main.css`, `dashboard.scss`
+
+## Enabling the Theme
+1. Log in as **administrator**.
+2. Go to **Administration → Configuration → Look and Feel → Themes**.
+3. Select your theme from the dropdown and save.
+
+## Notes
+- Chamilo 2.0 introduces a simplified and modular theme system.  
+- Legacy folders like `main/template/` and `main/css/` are no longer used.  
+- Refer to the [Chamilo 2.0 Developer Documentation](https://docs.chamilo.org/en/) for updates.
+
+> Note: This guide applies to Chamilo 2.0.  
+> For Chamilo 1.11.x, please see the previous version of this document.
+