Text size / Desktop & Mobile / Feature heading size

Hey, we'd like to adjust the text size for mobile devices. The headline size is perfectly fine for desktop and ipad, but it looks too big on mobile devices.

Question: How can we change the size so that it looks good on mobile devices?

We already asked Chat GPT and implemented a code in the custom.css file and activated the custom.css in the theme.liquid file but it seems not to work.

Any ideas?