Show text box on mobile view only

Hello,

I’d like to add a text box to a page of my site that only shows in the mobile view. I’ve already turned on the custom css field as described here: http://kriesi.at/documentation/enfold/turn-on-custom-css-field-for-all-alb-elements/ but I’m not sure what the css to use I’d like the text box to appear when the mobile menu appears (starting with tablets), all the way down to the smallest size. Thanks for the help!