Can I access the code with my admin
The template and CSS code cannot be accessed from the admin. You'll need to gain access to the git repository that (hopefully) exists with your site's code. 

Or in a less-preferable scenario, you could access the server via FTP and modify the templates and CSS there.

You might want to take a read through the Craft CMS docs as a starting point to get a little better idea of how the files are structured.