Hello Dario80,
If you want to replace the theme search with google search, you have to use plugin which are available on WordPress repo. Please find the best and suitable plugin and use. If you want that search on the place of theme search field, you have to write some custom Code and CSS for it too. Please try yourself if you are familiar with WordPress code. If you need any help, you can hire a customizer for it too. http://www.acmethemes.com/customization-request/
Best Regards,
Acme Supports