When you think about all the stuff that goes into what we think of as "Windows Phone 8.1," the reality is that a bunch of it isn't even part of the OS. There's the Cyan firmware update for Lumia users, of course, which adds a ton of new features of its own. There are various new and improved Microsoft and Nokia apps. And then there's a Live Lock Screen app that Microsoft promised to deliver all the way back in April. Well, it's finally here in beta form.

To be fair, Live Lock Screen (the app) does in fact technically take advantage of a new Windows Phone 8.1 "feature," in that this version of the OS has new APIs that make it possible for apps to customize the lock screen in previously impossible ways, by creating animated, interactive lock screen themes. For now, Live Lock Screen is the only app available that can access these APIs. But Microsoft says it will open up the APIs later so that any app maker can make their own apps that can change the lock screen theme.

Oddly, Live Lock Screen was the very first Windows Phone 8.1 "feature" that we saw when Joe Belfiore showed off the OS during the day one keynote address. He played around with the Live Lock Screen-customized lock screen before doing a short demo of the new Action Center.

But then he launches into the lock screen stuff.

"The second thing we've done to make Windows Phone 8.1 even more personal is a feature that lets you greatly personalize your lock screen," Mr. Belfiore said emphatically. "In Windows Phone 8, we had support so applications could put an image behind lock. But in Windows Phone 8.1, we've implemented a new set of APIs for the lock screen, which we're using (just) ourselves for a little while, before we publish them all to you. These APIs enable lock screen experiences that are highly interactive, where the application can take full control and really render whatever it would like."

"As the first app to use these (APIs), we've been collaborating with Rudy Huyn, star Windows Phone developer. He's been working with us to build out a first party experience which we, Microsoft, are going to publish in the store in the coming months to give users a wide range of highly varied lock screen themes."

Belfiore then showed off a few of the themes they had worked up. Flash forward almost four months, and now everyone with Windows Phone 8.1 can check out this app and customized their own lock screens according to the available themes.

A few notes about the app.

It's buggy. I've already had it crash twice, which sends the lock screen back to whatever design you configured in Settings, Lock Screen.

It's a standalone app. This app does not change the Lock Screen settings at all. There's no indication there that you're basically overriding whatever settings you make in that interface.

There are only a handful of themes. I don't think there's a single theme in the app I hadn't seen before. There are only six in total.

You can use the built-in style (kind of a blue sky with clouds) or use the Bing image of the day as the background, or you can pick from different backgrounds, including the stock Windows Phone lock screen backgrounds. That's a nice touch, and you can stack up different backgrounds and rotate through them on a schedule.

There isn't much variety. The themes are in fact static when you're looking at them. The "interactive" bit comes when you go to swipe the lock screen away: Each theme provides some kind of animated exit, and while some are kind of cool, most are simple fades.

As a proof of concept, this is a pretty good start. But I can't imagine why this took so long, nor can I explain why Microsoft has chosen to promote and work with an outside programmer on such a thing. The app is decent, or would be if it worked reliably. But I'm looking forward to getting some real star programmers on these APIs. I have a feeling we're going to see much better lock screen theme apps than this in the future.

You can download Microsoft Lock Screen Beta for Windows Phone 8.1 from the Windows Phone Store.