Right now, you have no custom.css file included in the
of your website. So, this means you’ve altered the inclusion of this CSS file within functions.php of the sample child theme came with the theme. And so it would make sense that putting anything in the file wouldn’t do anything.
This is what your functions.php of the sample Child theme should have looked like at the start. The framework is included and below that, the custom.css file is included.