Hello Naz Escote,
At the moment, we do not have an official template tutorial, but I can point you in the right direction here:
1. Go to "Template settings", and download the existing template.
2. Modify it on your computer.
3. Go to "Template addition", re-upload the tar. Give it a new name.
4. Go back to "Template Settings" and select the template you just uploaded.
When you refresh the site, you should see the changes you made!
I hope that helps!