Question: I'm new to Drupal Commerce.

I have created new product type and added custom fields everything and it's working fine.

I want to theme the Add a Product (form page). Please suggest me how to do that.

Thanks,
Selva

Answer: 
It depends what part of the page you want to theme, simple CSS, change the layout, or adjust the form.

To add CSS, you could attach your CSS file in `hook_page_build` for that path.

To change the page template, you could override the `page.tpl.php` file as `page--node--add--product.tpl.php`

You could also set a custom template in `hook_preprocess_page` by checking for the relevant path and setting `theme_hook_suggestions` to your template (e.g node/add/product and node/[nid]/edit)

Another option is using `hook_custom_theme` to specify a whole new theme for that specific path.

Lastly, you can theme the form itself using `hook_form_alter` and using either a template for the form, or adjusting the form elements there.
If you are afraid of codes, download the module "themekey" and install . then download the theme you want to use for the path. After that, go to the themekey admin link to set the theme for the specific path.