If you already use your Hugo project as a git repository, it is a better practice to use a theme as a submodule instead of using a git repository inside another git repository.
at the root of your hugo project : git submodule add 'the source' 'the destination'
such as an example : git submodule add https://github.com/cboettig/hugo-now-ui themes/hugo-now-ui
If your Hugo project is not a git repository then you simply execute git clone https://github.com/cboettig/hugo-now-ui themes/hugo-now-ui
Cut and past way
As you may notice Github offer you to download a zip file
Simply download it and extract the content inside ./themes/
I would recommand you to rename the directory from hugo-now-ui-master to hugo-now-ui
To discover the theme and build on top of it you must copy, as usual, the exampleSite at the root of your Hugo project. cp ./themes/hugo-now-ui/exampleSite/* ./
Again it’s a best practice to overwritten a files instead of modifying it. It help you to easily updating Hugo and the theme without loosing your customization. To learn more about customizing a Hugo themes