Developer is HTML template designed by 3rd Wave Media and converted for Grav by NunoPress LLC.
To install this template, just download the zip version of this repository and unzip it under /your/site/grav/user/themes. Then, rename the folder to developer.
You should now have all the theme files under
/your/site/grav/user/themes/developer
You can find inside theme directory the folder _demo where you can find the example contents for your site. If the theme is one-page layout copy all files inside _demo/pages into your home page folder. For the other folders follow the same name.