If you want to change your design you need to edit your .css file in your website root>App_Themes>DarkOrange>styles.css If you you want to edit product, category or manufacturer template then you need to edit .ascx files in Templates folder.
Thanks for your help. Is there is any way I can change the complete template. I want to change the complete look of template. I want navigation in different way. Actually I have different template for my new shopping cart site and want to implement in nopCommerce.
You need to edit the files as I write above, also if you want to change navigation or anything else you need to edit .ascx files in Modules folder. There is no other way to implement your style template.