Modify Your Header Theme
Go to Design → Edit Html/CSS in the left navigation, from here you will see a dropdown menu where you can modiy the header/footer html files. Along with the Css files for your template
If you are using the free Shared SSL (ex: https://yourstore.corecommerce.com) you may also have a secure_header.html which you will need to edit as well.
It's recommend to use HTTPS Everywhere. View our HTTPS set up guide by going here
We do also allow FTP access (Account->FTP) if you would like to add custom html pages, or custom css files. You will be able to do so from your FTP client of choice. Please see our FTP article for more details