To what extent can I change and customize the appearance of the Windows Phone interface? Are there built-in customization options or perhaps third-party apps that allow this?
You can change your accent color and background color (black or white) and lock screen image, as well as change your ringtone and other tones.
Beyond that, though, the UI paradigm of Windows Phone (use of live tiles on the home screen) is actually where we're expected to really personalize the phone. It's different than themes or desktop images, like on other phones or on computers, because the Windows Phone UI is intended to be very sparse and content-driven, without lots of UI chrome and fluff.
The start screen consists of any number of tiles, some of which (and increasingly, most of them) are live tiles. Each application that has a live tile often allows customization of its behavior and look&feel, as well. So, your personalization of your phone isn't surrounding the paradigm of color schemes and background images as much as it is centered around the content you want to look at on your phone.
You can get to your applications without dealing with the start screen, but you can choose what goes on there, and what live tiles you want to see. You can place and arrange the tiles on your start screen any way you want, and this layout can have a significant impact on your phone's look and feel and is a significant way of personalizing it.
However, of course the stock options available in Windows Phone aren't always enough. If you want to get particularly artistic and customize further, there are apps that let you specify completely custom tiles for your homescreen that can act as links to apps, websites, or perform other actions. One that I particularly like is called MegaTile. It can take an image and break it up into multiple start screen tiles. The screenshots are a good demonstration of what it can do.
So, that's what's available right now. Microsoft may add more in the future. It's definitely not quite what you may be used to on iOS or even Android, but it's not necessarily as limited as it may seem at first glance. I think people take the fact that live tiles are much more than simple static icons and your ability to organize your start screen for granted and forget that it can have a major impact on the look & feel of your phone.
P.S. Also what may be interesting to note about the lock screen image that not many people may be aware of is that it supports transparency. This is only available when you're not using a PIN code to lock the phone. All you need to do is download and choose a lock screen image with transparent sections and it will just work. I haven't done this for a good while but I have tried it myself and it did work.Tweet