The first thing you need to do is go to settings, about device and look at your model number. If you can post that back I'll be able to give you some more device specific info.

Rooting won't changed the look of your phone. It will only provide you with root access which gives you the opportunity to change the look of your phone.

You can unroot by flashing stock firmware.

 

That model is closest to the at&t version. I'm not 100% on rooting that model so I'll move your thread over to at&t all things root, where I'm sure others will be able to help you out.

 

How about my model number is GT-I9300?

 

IT is possible to un-root your phone, you just need to make sure that when you have not used titanium back up to delete any of the software/apps that you gained administrator access to through root. Like all those AT&T based programs, if you don't want them in the back ground, freeze them instead. When you what to unroot, unfreeze them.

The reason you may want to unroot is if you want to get any updates for your phone. However, if you rooted your phone and decided to get a ROM, then you can also get updates from the developer community.

Please message me if you have any further questions.

I have the samsung galaxy SIII from ATT with the same model # as you. I have guides and instructions that I have bookmarked that helped me through the process and how to not brick my phone or render it less user friendly.

Thanks