I am wanting to change my theme on Windows 7 to an unofficial theme but I have seen some people say it has messed with things on their computer or have just completely broke it. It looks fairly simple but i wanted some advice on it, Is there anything i should be aware of or any things i should do to prepare?

All the people i have seen doing it on YouTube seem to have nothing on their desktop and it looks like a brand new PC but mine isn't, Does that matter?

I have created a system restore point just in-case but I don't know if that's enough or not.