With this policy settings, you can provide a default experience for your users while still allowing each user to customize their theme. For example, if an end-user has already changed their theme, their setting will take precedence over a theme that is applied via Group Policy.
Download: Office 2013 Administrative Templates
In the Group Policy Editor, you can find this setting under:
User Configuration | Administrative Templates | Office 2013 | Global Options | Customize
To enable this setting, you must download the new ADMX files and import them in your environment, and ensure that the update found at http://support.microsoft.com/kb/2727096 is installed on client machines. For Office 365 ProPlus, this update is included in the August 2013 update, build 15.0.4517.1509 and later. Without the hotfix or the required build, this Group Policy setting will not have any effect.