Hi,

Yes, you can write your own HTML, CSS, and JavaScript code in Odoo without using any pre-existing templates. Odoo provides a built-in website builder that allows you to create and customize your own website pages using HTML, CSS, and JavaScript.

To get started, navigate to the Website module in Odoo and click on the "Edit Website" button. This will open up the website builder where you can create new pages and customize existing ones. From there, you can use the built-in text editor to write your own HTML, CSS, and JavaScript code and see the changes reflected in real-time.

Hope it helps