Hi Reading Taxis,
I am afraid you won’t be able to do it. WordPress is a CMS, which stands for Content Management System. To explain it simply, it is a system that allows you to create a site. It has it own configurations and any site you build using WordPress must abide by these standards and configurations.
HTML and CSS can be used to help build and improve a WordPress website, but you cannot get HTML and CSS code that built a whole stand-alone website and merge it in a WordPress site.
If you built a stand-alone site using HTML and CSS and you want to make it live in the internet, you have to upload it to a hosting account, but this will not be a WordPress site.
Regards,
Marcus