1. Enable Developer Mode: Go to your Odoo instance and enable developer mode. You can usually do this by appending ?debug to your Odoo URL.
2. Access User Interface: Once in developer mode, navigate to the user interface where you want to create the fixed vertical menu.
3. Customization: Click on the Edit button or navigate to the customization options, depending on your interface layout.
4. Edit XML or Templates: Depending on your familiarity and access level, you might need to edit XML files or templates to implement this change effectively. You'll typically find these under the views or templates directories in your Odoo custom module.
5. Modify Layout: Look for the layout files corresponding to the page layout where you want to add the fixed vertical menu. You'll need to add HTML/CSS code to create a fixed vertical menu on the left side of the page.
6. Implement Fixed Positioning: Utilize CSS to set the position of your vertical menu to fixed. This ensures that it remains fixed on the left side of the page regardless of scrolling.
7. Testing: After making changes, save your modifications and test the page to ensure that the fixed vertical menu appears correctly and functions as expected.
8. Refinement: Fine-tune the appearance and behavior of your fixed vertical menu as needed, considering factors like responsiveness and usability.
9. Deployment: Once satisfied with the changes, deploy your modifications to your production environment for all users to access.

Thank's, Niyas, but backend themes might not work with Odoo Enterprise Edition.

Thank's for all!

Hi,
You can download and install this theme module as it is or refer to the coding done in it:  https://apps.odoo.com/apps/themes/17.0/muk_web_theme/

Thanks