Hi,

Please go to Customize > Addtional CSS then add custom css to do that:

#page.site {
    transform: none;
    background: #fff;
    max-width: 1400px;
    display: block;
    overflow: hidden;
    margin: 0 auto;
    box-shadow: 0 5px 8px rgba(0,0,0,0.2);
    -webkit-box-shadow: 0 5px 8px rgba(0,0,0,0.2);
    -moz-box-shadow: 0 5px 8px rgba(0,0,0,0.2);
    background-repeat: repeat;
    background-attachment: fixed;
    background-size: cover; 
}
#page.site .site-header {
    max-width: 1400px;
}

Best Regards,

PenciDesign