Odoo themes are more complicated than a simple set of HTML pages or a simple dynamic page that uses JavaScript. Odoo's theme framework is built around blocks in which you can use drag and drop to assemble both your page structure and your page content. Before you start trying to create your own themes make sure you are thoroughly comfortable with the basics of creating pages in Odoo's Website Builder.

If you are serious about developing themes, then you will need to understand the main technologies that make up the framework. These include Bootstrap, jQuery, jQuery UI, and underscore.js. Also, like other Odoo development tasks, you will gain a lot of insight by examining the existing themes that are included with a basic Odoo installation.