If you want to customize your free legacy Blueprint themes that have not been customized than you can follow this tutorial for the same. When you have to create the a custom layout for your categories, product pages or web pages a unique look and feel you have to create a custom layout. This can be beneficial if the group of products that you have needs a different layout than what you have. Please follow these steps to understand how you can create a custom file layout based on your default layout themes.
Creating the custom layout Template files
For creating the custom layout template files, please go to the store front designs, then go to my themes and then click on edit HTML/CSS
On the left side, you have list of other template files, select category.html for a custom category layout, product.html for a custom page layout or page.html for a custom webpage layout.
In Order to edit the content, copy all the contents of the file and paste it on a plane text editor on your computer, e.g. Notepad or textedit
Once you are done in making the changes as per your requirements, save the file with a new name. The format of the file name should begin with a “_” and end with .html
Uploading the new file on the BigCommerce backend, it must be uploaded via WebDAV. Once you are connected to the WebDAV, upload your file to the template folder.
For assigning your custom layout file to a category, go to the products and then to product categories, then click on action > Edit to the right of the category that you want to assign the file to
Scroll to Template Layout file dropdown and select your custom file and the save your changes