How to do 2 responsive columns without table with Persona Theem

i tried; probably not hard enough, by adding this code to a page

https://www.w3schools.com/howto/howto_css_two_columns.asp

then this in the Custom CSS

.row {
  display: flex;
}

.column {
  flex: 50%;
}

which work 

but then when I want to add the responsiveness

/* Responsive layout - when the screen is less than 600px wide, make the two columns stack on top of each other instead of next to each other */
@media screen and (max-width: 600px) {
  .column {
    width: 100%;
  }
}

it doesn’t stack