Closing an application or restarting it in Windows Phone 8?

From what I've been able to see on my Lumia 920, there's no obvious way to restart an application or force-close it.

The official Twitter application has a setting to change the theme from light to dark, but says that changing the theme won't actually occur until the application is restarted. So how do I restart it? The only way I've found that works so far is to reboot the entire phone, but I can't imagine that they intend for people to have to do that in order to change a simple setting.

Is this functionality present?