you can visualize the code generated by WebRatio by right clicking on the model element (e.g. a Page, an Entity, a Unit,...) and choose Open Generated Code command. For example, in case of a Page you can open the generated JSP page. You can modify the generated files, but the changes will be overwritten at the next generation.
To see the Html code of page inside the browser you can right click on the page opened in the browser and choose the view source page command.
To customize the style of an element (e.g. a Page) you have to change its Layout template. For more details about style you can refer these articles: Create a custom template, Getting started with the Style Project, Create a Style Project from a mockup.