The hugoThemes site is on Github, you can also find the build script and more information there: GitHub - gohugoio/hugoThemes: A curated directory of Hugo themes
On the Hugrid theme page, click on the ‘Homepage’ button. That brings you to Github.
There click on the green button to the right that says ‘clone or download’.
That click brings up a pull-down menu; select ‘Download Zip’ there.