I've bought your theme today to have a fast start with a basic site but ran into some problems during a default and clean installation. I'm willing to debug them myself but maybe its good to dive into it so you can smoothen the experience for other users.
Did a clean installation of October (latest) plus your theme. Installed al necessary plugins and everything is updated to the latest version. All demo pages are created in the backend.
Front end: An exception has been thrown during the rendering of a template ("count(): Parameter must be an array or an object that implements Countable") in "/.../public_html/themes/ketikidis-octobercms-success/layouts/static.htm" at line 11.
back end: When opening one of the pages (pages > pages > Home) in the backend it gives me an error: "Unable to find configuration file themes/success/meta/static/blocks.yaml defined for Backend\FormWidgets\Repeater." on line 63 of /.../public_html/modules/system/traits/ConfigMaker.php