Button “returns to the top” invisible

Hi there,
the back to top button id hidden by design for very small devices (smartphones) with CSS.
You can change this behavior by overriding the theme CSS in your child-theme (if you have one) or in Apperance -> Customize -> Advanced options -> Additional CSS

some useful docs about CSS:
https://docs.presscustomizr.com/article/72-basics-of-css-and-html-for-wordpress-themes
https://docs.presscustomizr.com/article/272-inspect-your-webpages-in-your-browser-with-the-development-tools
https://docs.presscustomizr.com/article/63-css-resources-for-wordpress-developers

Very interesting.
Since I am not a CSS expert, could you tell me the code to insert in the Additional CSS in order to always display the button on a smartphone?

Thank you very much!

Andrea.