Yes you can. You can access Settings>Templates where you can edit the existing templates. You have a visual editor to change colours, fonts and images of the current templates. If you wish to import or create a new one, you can edit the HTML. To do so, please follow the instructions under the link above.
In order to use your template, please set it as default here.
We have a paid service to build and edit your template. If you wish us to help you with the process, please contact us by email at email@example.com. The price of the service may vary according to the changes you would like us to implement.