Donc le code...
Dans le header.php
<meta name="viewport" content="initial-scale = 1.0, maximum-scale = 1.0, user-scalable = no, width = device-width">
...
<link rel="stylesheet" href="<?php bloginfo('stylesheet_url') ?>" media="screen" />
...
<div id="art-main">
<?php if(theme_has_layout_part("header")) : ?>
<header class="clearfix art-header<?php echo (theme_get_option('theme_header_clickable') ? ' clickable' : ''); ?>"><?php get_sidebar('header'); ?></header>
<?php endif; ?>
dans le style.css
#art-main
{
   background: #EFEFEF;
   margin:0 auto;
   font-size: 13px;
   font-family: Ubuntu, Arial, 'Arial Unicode MS', Helvetica, Sans-Serif;
   font-weight: normal;
   font-style: normal;
   position: relative;
   width: 100%;
   min-height: 100%;
   left: 0;
   top: 0;
   cursor:default;
   overflow:hidden;
}
header, footer, article, nav, #art-hmenu-bg, .art-sheet, .art-slidenavigator > a, .art-checkbox:before, .art-radiobutton:before
{
   display: block;
   -webkit-box-sizing: border-box;
   -moz-box-sizing: border-box;
   box-sizing: border-box;
}
.art-header
{
   border:1px solid #C779BB;
   margin:20px auto 0;
   height: 250px;
   background-image: url('images/header2.jpg');
   background-position: left top;
   background-repeat: no-repeat;
   position: relative;
   min-width: 1025px;
   max-width: 1536px;
   width: 80%;
   z-index: auto !important;
}
.responsive .art-header 
{
   background-image: url('images/responsive2.jpg');
   background-position: left center;
}
et pour finir le code du responsive, style.responsive.css
.responsive body
{
   min-width: 240px;
}
  
.responsive .art-content-layout img,
.responsive .art-content-layout video
{
   max-width: 100%;
   height: auto !important;
}
.responsive.responsive-phone .art-content-layout img 
{
   float: none !important;
}
    
.responsive .art-content-layout .art-sidebar0, 
.responsive .art-content-layout .art-sidebar1, 
.responsive .art-content-layout .art-sidebar2 
{
   width: auto !important;
}
    
.responsive .art-content-layout, 
.responsive .art-content-layout-row, 
.responsive .art-layout-cell 
{
   display: block;
}
    
.responsive .image-caption-wrapper 
{
   width: auto;
}
.responsive.responsive-tablet .art-vmenublock,
.responsive.responsive-tablet .art-block
{
   margin-left: 1%;
   margin-right: 1%;
   width: 48%;
   float: left;
   -webkit-box-sizing: border-box;
   -moz-box-sizing: border-box;
   box-sizing: border-box;
}
.responsive .art-responsive-embed 
{
   position: relative;
   padding-bottom: 56.25%;
   /* 16:9 */
	height: 0;
}
.responsive .art-responsive-embed iframe,
.responsive .art-responsive-embed object,
.responsive .art-responsive-embed embed
{
   position: absolute;
   top: 0;
   left: 0;
   width: 100%;
   height: 100%;
}
.responsive .art-header 
{
   width: auto;
   height: auto;
   min-height: 150px;
   min-width: 1%;
   margin-top: 1%;
   margin-right: 1% !important;
   margin-left: 1% !important;
   background-position: left center !important;
   background-size: cover !important;
   background-repeat: repeat !important;
}
.responsive .art-header .art-headline, 
.responsive .art-header .art-slogan 
{
   display: block !important;
   top: 0 !important;
   left: 0 !important;
   margin: 2% !important;
}
    
.responsive .art-header .art-headline a, 
.responsive .art-header .art-slogan 
{
   white-space: normal !important;
}
    
.responsive .art-header .art-shapes .art-textblock 
{
   padding: 2% !important;
   margin: 2% !important;
   line-height: normal;
   background-image: none;
   background-color: rgba(127, 127, 127, 0.7);
   color: #eee !important;
   display: none;
   top: auto;
   left: auto;
   width: auto;
   height: auto;
}
.responsive .art-header .art-shapes .art-textblock *,
.responsive .art-header .art-shapes .art-textblock a:hover,
.responsive .art-header .art-shapes .art-textblock a:visited,
.responsive .art-header .art-shapes .art-textblock a:link 
{
   color: #eee !important;
}
    
.responsive .art-header .art-shapes .art-textblock>div 
{
   display: none;
   top: auto;
   left: auto;
   width: auto;
   height: auto;
   padding: 0 !important;
}
.responsive .art-header .art-shapes,
.responsive .art-header .art-shapes *
{
   position: relative;
   text-align: center;
   -webkit-transform: none !important;
   -moz-transform: none !important;
   -o-transform: none !important;
   -ms-transform: none !important;
   transform: none !important;
}
.responsive .art-header #art-flash-area,
.responsive .art-header .art-shapes>*
{
   display: none;
}
.responsive #art-header-bg 
{
   background-position: center center !important;
   background-size: cover !important;
   background-repeat: repeat !important;
}
...
@media all and (max-width: 800px)
{
    #art-resp, #art-resp-t { display: block; }
    #art-resp-m { display: none; }
}
@media all and (max-width: 480px)
{
    #art-resp, #art-resp-m { display: block; }
    #art-resp-t { display: none; }
    .responsive .art-header { display: none; }
}