Vantage theme is not mobile Responsive ?

hello,
i am using vantage theme… but its not mobile responsive… the logo and menu bar are overriding the slider images… how to fix this issue…!! i have used custom code for full screen slider.. wch works well on the web… but not on mobile. also the size of the slider is too small in mobile device.. need to fix this issue…

i have used this CSS :

.site-header {
position: absolute;
left: 0;
right: 0;
}

.hgroup, .main-navigation {
z-index: 400;
}

.main-navigation {
background: none !important;
}

#main.site-main {
padding: 0 35px !important
}