Where do I need to place the code to fix not fitting banner?

Hello,

I’m making a new homepage with the theme debut, but my banner (element: photo with textoverlay) doesn’t fit the mobile perspective. I then contacted Shopify Support, they referred me to this forum: https://community.shopify.com/c/shopify-design/how-do-i-change-the-banner-size-on-mobile-view-debut-theme/td-p/1086742 .
So I had to add a code, but I don’t know where to do this. Shopify Support was unable to help me with this either. They recommended creating a forum.
On the referenced forum it says that the code must be added at the asset → theme.scss.liquid. But I don’t have it in my codes. I only have theme.css. Now it appears that scss or scss.liquid is no longer supported by some themes, so that css is now used for this. So I had added the code at the bottom in theme.css, but it didn’t help either. Where am I doing something wrong?

Here are some photos:

The banner on full screen:

The banner on mobile view:

Thanks in advance.

Kind regards,
Bedruktvoorjou.nl