How to work with @media ?

Problem/Motivation

Hello,

I enjoy playing with your theme. :) I would like to use different blocks for a wide screen and a mobile screen:

@media only screen and (max-width: 700px) {
#block-slogan1wide-3, #block:block=slogan1wide_3 {
    display: none !important; }
}

@media only screen and (min-width: 450px) {
#block-slogan2mobiel-3, #block:block=slogan2mobiel_3 {
    display: none !important;}
 }

This works with the DXPR theme but not with the d8w3css theme.

How can I make this work?

I am looking forward to your answer :)

Jippie.