would like to use my child theme to use CSS. I unfortunately don’t get it activated so that it reads my CSS. I need the right code for the function.php i think.
If you want just to add CSS you do not need the child theme. You can add your CSS to the Theme Settings > Custom CSS. It would be safe even after udpdates.
The child theme is necessary when you custom PHP code of the theme.
If you want a correct and safe CSS application you would better use Theme Settings > Custom CSS, please consider the option to insert comments into code to avoid confusion.