Navigation: Store Design>Templates
Here you will see all the available template options.
You can either Apply theme or View Demo of the store from here.
View Demo will open the demo store in a New tab.
To apply a theme click on Apply Theme button, you will get a pop-up notification as below:
Once you click on Apply & confirm, the selected theme/template will be applied on your store.
You can now see it live on your store.