In new apps the css goes under settings → appearance. You could give the rich text component a class name under options or directly in the html itself.
One way to keep a 2 column container in both desktop and mobile is to give a 2 column container the class name grid under the options for the container.
Then under settings → appearance use this css
.grid .oneToOne {
grid-template-columns: repeat(2, minmax(0px, 1fr));
}
Hat tips to @Himaladin. Look out for anything that guy posts about css he’s very good.