Ghacks posted a tutorial about installing unsigned themes in Windows 8

While you have some versatility when it comes to themes in Windows 8, you can’t just go ahead and install third party themes in it. Microsoft is protecting the system from that, and if you are craving for themes that change more than some colors and background wallpapers, you do need to look for third party themes for that.

Before you can do so though you need to patch the three system files themeui.dll, UXInit.dll and uxtheme.dll on Windows 8. Do not worry though, there are tools out there that can patch those files for you. I suggest you download and install UxStyle which is available for Windows 8 in an experimental version. An alternative to that is Ultra UX Theme Patcher.

UxStyle installs itself as a service that modifies the three Windows system files in RAM. What this means is that that the files remain untouched on the hard drive so that they can be updated via Windows Update whenever Microsoft releases updates for them.

  How to install unsigned themes in Windows 8