A theme hosted on GitHub instead of the WordPress.org directory installs the same way as any theme you upload by hand. You have 2 routes.
Upload the zip
- Download the theme’s release zip from its GitHub releases page.
- In wp-admin, go to Appearance, Themes, Add New, Upload Theme.
- Choose the zip, install, and activate.
Use the release zip where one is offered. The generic Download ZIP button on the green Code menu wraps the files in a folder named after the branch, which can leave the theme in a folder like theme-main. A release zip avoids that.
Clone the repository
If you work from the command line, clone the repository straight into wp-content/themes and activate the theme under Appearance, Themes.
About updates
A theme installed this way doesn’t update through the WordPress.org directory. Some themes bundle their own update checker so notices still appear in wp-admin. Loupely Canvas does this, and its release zip is built to install cleanly. For how that works, see the article on keeping a GitHub-hosted theme updated.
loupely-canvas.zip asset.