To do this click on the settings button for Add/Edit Layouts. You will see the basic layout which is predefined. You are also not able to edit this layout. You can create a new one by clicking on the Add new button.
In the new screen you need to fill in a Layout name and a Width (best looking is 250) When you change a field you will automatically see how it will look like in the store next to the input fields. When you created a layout you like you can save the layout by using the Create / Save button.
Now all you need to do is edit/create a store in Add/Create Store and select your newly created layout.
The second way to change how the site looks is by using a CSS file. You can enable the usage of a CSS file in the Plugin Settings
If you want to see some examples of pre created layouts go to here
If you created a new layout you would like to get implemented in the plugin let me know and i will see if i can include it as an option.
You can also share css adjustments in below comments for other plugin users.