1. Follow the tutorial Theme - How to Apply Custom Themes
, download the patches from there . Run the appropriate patch. If the system files are patched already, it will say that the files are patched .
2. Just as the same of the first answer ... use the patch listed there in the tutorial . It wount create any problem .
3. Do's : Only one thing ... have an image backup of your system before applying patches or replacing system files , so that you can get your things back if the situation goes really critical . Dont's : Dont apply a theme with an installer that runs an adware .
4. themeservice.dll , themeui.dll and uxtheme.dll .
If you want to replace system files like explorer.exe, explorerframe.dll etc , you are to take ownership of those files .