I'm writing this to explain what root and ROM's are to try and help users understand.
I get many comments about why they can't install theme chooser theme's on their device so hopefully this will help. Going to put it in the simplest terms that I can.
This is a process that can be done to your Android device to basically give you superuser permissions. When you first buy and Android device you are not able to make changes to system files and certain other things like that. Rooting gives you the ability to be able to do those things.
Think of it like this. On a PC you are able to see your C:\Windows\System32 folder. Imagine that being locked down and not being able to access it. The normal user does not need to change those files because it could damage the system and cause it not to work properly. Well, that is what Android does on all its devices. They lock down system files to keep users from alter system files. Why? Because if a user that didn't know what they were doing was to alter something they shouldn't they would be calling the manufacture demanding they get a new phone because its not working. When all along it was the users fault for altering the device.
So, why would you want to root? Well if you know what your doing or read some tutorials root can be very useful. For one if your device is rooted you can install a custom recovery and install ROM's to your device. But we'll get to that in just a minute. Other reasons to root is to allow apps from Google Play to run properly. For example a app that backs up all your apps and app data on your device needs root to be able to do that. That comes in handy.
What are ROM's? Well think of it this way. A ROM is basically like an operating system. When a manufacturer like HTC or Motorola develops a device they take Google's Android source as a base then build their own works on top of that. That is a ROM. When you get your device and turn it on for the first time your are seeing the manufacturer's ROM. In most cases it looks very similar from one device to the next, but some look very different. If you've ever heard of the Nexus line of devices, those are pure Google. They don't contain any other things built into it but what Google has developed.
So why do you want a custom ROM? Custom ROMs make a device very close to pure Google. They don't add all the extras (bloatware) that manufactures pile on top. The most popular ROM's are CyanogenMod and AOKP (Android Open Kang Project). Both of these ROM's support theme chooser themes.
What's a theme chooser theme? That is something that these custom ROM's have added to their systems to allow developers to create themes to put on Google Play (or othre places) and let users theme their entire device. Theme chooser themes allow dev's to skin almost everything you see on your device. It makes it very simple to change between themes as well.
What ROM's are compatible with theme chooser theme's? That is up to the developer of the ROM. There are many custom ROM's out there that do not include theme chooser support. You can email the developer or check where they have posted to ROM to see if they support it. If your ROM does support theme chooser themes there will most likely be and option under system settings for themes.
How do you get a custom ROM? That depends on the device you have. The largest forum i know of it XDA -Developers. If you go there and search your device there are many how to's to be able to get rooted, install a custom recovery and finally a custom ROM.
What s custom recovery? Every Android device that is made has a recovery option. This is something the manufacturer uses to allow users to update their device and other things. A custom recovery is something that a 3rd party developer creates to allow you to fully utilize recovery. The manufacturer only allows certain function in a stock recovery, but a custom recovery allows much more. Making a backup of your device you can restore at a later time or in case something messes up on device. It allows you to flash custom ROM's and zip files that can do various things for you. They also have many more functions.
There are some risks when doing these things. Some manufacturers void warranties, you could turn your device into a brick (meaning its good for nothing but a paperweight) and so on. If you read and ask questions that is likely not going to happen. If have been rooting and installing custom ROM's for about 4 years now and I have never bricked a device. Just do your research and you'll be fine.