Change the structure of the Category Blog

Hello i want to change the structur of the category blog - article page in making a second option available at the category- option - layout field.

Which source php do i have to copy and adjust. Are there any further changes nessecary?
Thanks
Oliver

For custom layout category blog, you can copy files php in folder: [root]/plugins/system/t4/themes/base/html/com_content/category/

to folder 'local' (create this folder if it doesn't exist) in template:
[root]/templates/ja_stark/local/html/com_content/category/

Let us know if you need any further help.