How to remove the (yellow) background from AdSense ads?

To fix it add the following css rule in your theme options or style.css:

ins.adsbygoogle { background: transparent !important; }

But how can I add to the Child-theme?

Hi @jpbenfica,

I hope you are well today and thank you for your question.

You can use the CSS code on your Travelify site by adding it in the Custom CSS option of your theme on the below path.

Admin Area -> Apperance -> Customize -> Travelify Other Options

Or you can add it in the style.css file of your child theme.

Best Regards,