After you choose one you like, then you will need to install and activate the theme.

Here's how:

When you click on a theme, it will look like this. Install will be at the top left of the theme as shown:

Click install! Once you click install, the button will change to activate as shown below:

Then you can click back to themes and the themes you just installed will look like so:

Now just simply click activate. And your theme will be activated. Watch:

Before Activated:

After Activated: