The new Theme feature is something that we were waiting for a long time. Before if you need to change the theme you need to do some Javacript, change XML, re-write code and was something not supported by Microsoft, so this will help us configure how the client want. But the use of the preview its a pain, therefore this preview solution made by MVP Guido Preite, will help us :
With this tool you can simple export and then import into CRM and publish, bear in mind the logo since is a web resource you need to upload the image yourself.
Here an example of theme changed: