To add custom designs to your presentation, you must first save the presentaiton to disk. After that, you can clone existing designs to your presentation. When you clone a design to your presentations, a folder called designs will be created.

After you press the clone button, you will be asked to provide an ID in the form [domain name]/[design name], e.g. example.com/my_simple.

After that, the design is created for you. You can add additional images and other files to your design. Your folder structure should then look like this:
[parent folder]
+--- [presentation name].md
\--- designs
\--- example.com
+--- my_simple
+--- dragonfly-light.jpg
+--- dragonfly.jpg
+--- design.css
+--- design.json
\--- preview.md
Once you have done this, you can select the example designs from your marmota.app presentation:

You can start to edit your custom design from the design selector:

After that, learn how to edit your own desings using the design editor