From the given list of themes, you can select on Import Theme to add it to the list or click on Explore all themes for more themes. It takes you to the StoreHippo theme page where there are plenty of themes to choose from.
You can click on View Demo for the demo of the theme. Click on Theme Details, the following page appears:
Click on Import Theme and the theme will be imported to your store.
In order to make it as a default theme, click on Make Default Theme from the Actions.
The theme is applied to your store and it would appear under Active themes.