I used the theme Quark (Default Theme) and it shows on the preview of a picture (Mountains) as the background of the homepage. It isn’t showing up on my version of the website.
Hi @Bitmap, welcome! If you just installed the theme on an existing Website you likely do not have the page content as shown. You can download a skeleton of Quark, which contains both the theme and sample content etc.
https://getgrav.org/download/skeletons/onepage-site/2.0.0
https://getgrav.org/download/skeletons/blog-site/2.0.0
By exploring these two site you can also learn where files for certain site aspects need to be etc.