Ever since Microsoft introduced the theming engine in Windows XP, they don't allow visual styles (themes) which are not digitally signed by themselves to be used. Windows 8 is no different in that regard, so we need to patch certain system files to be able to use these themes. In this tutorial, I will show you how you can use third party themes:
Option 1 (Recommended): UxStyle
Install the UxStyle software developed by Rafael Rivera. The main benefit of this app is that it keeps system files untouched on disk and works transparently in memory.
Just follow the instructions in the installation wizard.