Go to your wordpress admin dashboard, choose appearance, and then click themes. (Appearance > Themes)
On the themes page, click add new on top of the page and choose the Lunar Theme file on your computer
Here are some simple ways to install and activate themes on your wordpress site
https://www.wpbeginner.com/beginners-guide/how-to-install-a-wordpress-theme/
https://winningwp.com/how-to-install-and-set-up-a-new-wordpress-theme/