How to add a new theme? I am trying to add in localhost.
Take a look at magento docs for info http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/theme-apply.html
Like @Martins said: The official Magento documentation can help you.
Also, look at the default blank/luma themes. Looking at 'how Magento does it' is always very insightful on how you can achieve similar results yourself.