Download the .zip file.
Copy and paste or extract the contents onto your desktop.
Go to Admin CP > Appearance > Styles > Import > Choose file > Import
Do not make custom edits to this file, instead create a child style and make edits in that. To do this:
Go to Admin CP > Appearance > Styles > Add style > Fill it out > Parent style > AdminIntel-Style > Save
Go to Admin CP > Appearance > Templates > Search: extra.less
Copy & paste: