CSS for smartphone versions

Good morning:

I am having trouble styling the mobile version of my forms:

This is how it looks on desktop:

https://mentorclasesparticulares.com/wp-content/uploads/2023/03/capture1.jpg

And this is how it looks on a smartphone:

https://mentorclasesparticulares.com/wp-content/uploads/2023/03/capture2.jpg

I’d like to delete the solid border (I’ve tried to do so by using CSS, but I don’t know what I’m doing wrong, it works on Google > Inspect but not when I add the line to Customize > Custom CSS) and to get the buttons to work in columns, one next to each other.

If you can help me with this, thanks in advance!!

Cinthya

The page I need help with: [log in to see the link]