Default child theme

Hi there, My blog https://blog.frank-muennich.net uses catch-flames-pro-child. While additions to the functions.php (shortcodes e.g.) work very well any additional css has no effect whatsoever. So I was unable to move my custom css from the customizer to the child-theme. I have even tried to modify the functions.php according to the advice given in the codex (https://codex.wordprerss.org/child_themes) without success. Could you please help. Best regrads Frank