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.
By exploring these two site you can also learn where files for certain site aspects need to be etc.